Mailman 3 July 1999 - Python-announce-list

Python Language FAQ - Section 4
by Markus Fleck July 7, 1999

July 7, 1999

This FAQ newsgroup posting has been automatically converted from an HTML snapshot of the original Python FAQ; please refer to the original "Python FAQ Wizard" at <http://grail.cnri.reston.va.us/cgi-bin/faqw.py> if source code snippets given in this document do not work - incidentally some formatting information may have been lost during the conversion. ---------------------------------------------------------------------------- The whole Python FAQ - Section 4 Last changed on Mon Jun 28 19:36:09 1999 EDT (Entries marked with ** were changed within the last 24 hours; entries marked with * were changed within the last 7 days.) ---------------------------------------------------------------------------- 4. Programming in Python 4.1. Is there a source code level debugger with breakpoints, step, etc.? 4.2. Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)? (Also phrased as: Can I use a built-in type as base class?) 4.3. Is there a curses/termcap package for Python? 4.4. Is there an equivalent to C's onexit() in Python? 4.5. When I define a function nested inside another function, the nested function seemingly can't access the local variables of the outer function. What is going on? How do I pass local data to a nested function? 4.6. How do I iterate over a sequence in reverse order? 4.7. My program is too slow. How do I speed it up? 4.8. When I have imported a module, then edit it, and import it again (into the same Python process), the changes don't seem to take place. What is going on? 4.9. How do I find the current module name? 4.10. I have a module in which I want to execute some extra code when it is run as a script. How do I find out whether I am running as a script? 4.11. I try to run a program from the Demo directory but it fails with ImportError: No module named ...; what gives? 4.12. I have successfully built Python with STDWIN but it can't find some modules (e.g. stdwinevents). 4.13. What GUI toolkits exist for Python? 4.14. Are there any interfaces to database packages in Python? 4.15. Is it possible to write obfuscated one-liners in Python? 4.16. Is there an equivalent of C's "?:" ternary operator? 4.17. My class defines __del__ but it is not called when I delete the object. 4.18. How do I change the shell environment for programs called using os.popen() or os.system()? Changing os.environ doesn't work. 4.19. What is a class? 4.20. What is a method? 4.21. What is self? 4.22. What is an unbound method? 4.23. How do I call a method defined in a base class from a derived class that overrides it? 4.24. How do I call a method from a base class without using the name of the base class? 4.25. How can I organize my code to make it easier to change the base class? 4.26. How can I find the methods or attributes of an object? 4.27. I can't seem to use os.read() on a pipe created with os.popen(). 4.28. How can I create a stand-alone binary from a Python script? 4.29. What WWW tools are there for Python? 4.30. How do I run a subprocess with pipes connected to both input and output? 4.31. How do I call a function if I have the arguments in a tuple? 4.32. How do I enable font-lock-mode for Python in Emacs? 4.33. Is there a scanf() or sscanf() equivalent? 4.34. Can I have Tk events handled while waiting for I/O? 4.35. How do I write a function with output parameters (call by reference)? 4.36. Please explain the rules for local and global variables in Python. 4.37. How can I have modules that mutually import each other? 4.38. How do I copy an object in Python? 4.39. How to implement persistent objects in Python? (Persistent == automatically saved to and restored from disk.) 4.40. I try to use __spam and I get an error about _SomeClassName__spam. 4.41. How do I delete a file? And other file questions. 4.42. How to modify urllib or httplib to support HTTP/1.1? 4.43. Unexplicable syntax errors in compile() or exec. 4.44. How do I convert a string to a number? 4.45. How do I convert a number to a string? 4.46. How do I copy a file? 4.47. How do I check if an object is an instance of a given class or of a subclass of it? 4.48. What is delegation? 4.49. How do I test a Python program or component. 4.50. My multidimensional list (array) is broken! What gives? 4.51. I want to do a complicated sort: can you do a Schwartzian Transform in Python? 4.52. How to convert between tuples and lists? 4.53. Files retrieved with urllib contain leading garbage that looks like email headers. 4.54. How do I get a list of all instances of a given class? 4.55. A regular expression fails with regex.error: match failure. 4.56. I can't get signal handlers to work. 4.57. I can't use a global variable in a function? Help! 4.58. What's a negative index? Why doesn't list.insert() use them? 4.59. How can I sort one list by values from another list? 4.60. Why doesn't dir() work on builtin types like files and lists? 4.61. How can I mimic CGI form submission (METHOD=POST)? 4.62. If my program crashes with a bsddb (or anydbm) database open, it gets corrupted. How come? 4.63. How do I make a Python script executable on Unix? 4.64. How do you remove duplicates from a list? 4.65. Are there any known year 2000 problems in Python? 4.66. I want a version of map that applies a method to a sequence of objects! Help! 4.67. How do I generate random numbers in Python? 4.68. How do I access the serial (RS232) port? 4.69. Images on Tk-Buttons don't work in Py15? 4.70. Where is the math.py (socket.py, regex.py, etc.) source file? 4.71. How do I send mail from a Python script? 4.72. How do I avoid blocking in connect() of a socket? 4.73. How do I specify hexadecimal and octal integers? 4.74. How to get a single keypress at a time? 4.75. How can I overload constructors (or methods) in Python? 4.76. How do I pass keyword arguments from one method to another? 4.77. What module should I use to help with generating HTML? 4.78. How do I create documentation from doc strings? 4.79. How do I read (or write) binary data? 4.80. I can't get key bindings to work in Tkinter 4.81. "import crypt" fails 4.82. Are there coding standards or a style guide for Python programs? 4.83. How do I freeze Tkinter applications? 4.84. How do I create static class data and static class methods? 4.85. __import__('x.y.z') returns <module 'x'>; how do I get z? 4.86. Basic thread wisdom 4.87. Why doesn't closing sys.stdout (stdin, stderr) really close it? 4.88. What kinds of global value mutation are thread-safe? 4.89. How do I modify a string in place? 4.90. How to pass on keyword/optional parameters/arguments ---------------------------------------------------------------------------- 4. Programming in Python ---------------------------------------------------------------------------- 4.1. Is there a source code level debugger with breakpoints, step, etc.? Yes. Check out module pdb. It is documented in the Library Reference Manual; pdb.help() also prints the documentation. You can write your own debugger by using the code for pdb as an example. Pythonwin also has a GUI debugger available, based on bdb, which colors breakpoints and has quite a few cool features (including debugging non-Pythonwin programs). The interface needs some work, but is interesting none the less. A reference can be found in http://www.python.org/ftp/python/pythonwin/pwindex.html Richard Wolff has created a modified version of pdb, called Pydb, for use with the popular Data Display Debugger (DDD). Pydb can be found at http://daikon.tuc.noao.edu/python/, and DDD can be found at http://www.cs.tu-bs.de/softech/ddd/ ---------------------------------------------------------------------------- 4.2. Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)? (Also phrased as: Can I use a built-in type as base class?) No, but you can easily create a Python class which serves as a wrapper around a built-in object, e.g. (for dictionaries): # A user-defined class behaving almost identical # to a built-in dictionary. class UserDict: def __init__(self): self.data = {} def __repr__(self): return repr(self.data) def __cmp__(self, dict): if type(dict) == type(self.data): return cmp(self.data, dict) else: return cmp(self.data, dict.data) def __len__(self): return len(self.data) def __getitem__(self, key): return self.data[key] def __setitem__(self, key, item): self.data[key] = item def __delitem__(self, key): del self.data[key] def keys(self): return self.data.keys() def items(self): return self.data.items() def values(self): return self.data.values() def has_key(self, key): return self.data.has_key(key) A2. See Jim Fulton's ExtensionClass for an example of a mechanism which allows you to have superclasses which you can inherit from in Python -- that way you can have some methods from a C superclass (call it a mixin) and some methods from either a Python superclass or your subclass. See http://www.digicool.com/papers/ExtensionClass.html. ---------------------------------------------------------------------------- 4.3. Is there a curses/termcap package for Python? [Andrew Kuchling] The standard Python distribution comes with a curses module in the Modules/ subdirectory, though it's not compiled by default. However, that module only supports plain curses; you can't use ncurses features like colors with it (though it will link with ncurses). Oliver Andrich has an enhanced module that does support such features; there's a version available at http://andrich.net/python/selfmade.html#ncursesmodule . ---------------------------------------------------------------------------- 4.4. Is there an equivalent to C's onexit() in Python? Yes, if you import sys and assign a function to sys.exitfunc, it will be called when your program exits, is killed by an unhandled exception, or (on UNIX) receives a SIGHUP or SIGTERM signal. ---------------------------------------------------------------------------- 4.5. When I define a function nested inside another function, the nested function seemingly can't access the local variables of the outer function. What is going on? How do I pass local data to a nested function? Python does not have arbitrarily nested scopes. When you need to create a function that needs to access some data which you have available locally, create a new class to hold the data and return a method of an instance of that class, e.g.: class MultiplierClass: def __init__(self, factor): self.factor = factor def multiplier(self, argument): return argument * self.factor def generate_multiplier(factor): return MultiplierClass(factor).multiplier twice = generate_multiplier(2) print twice(10) # Output: 20 An alternative solution uses default arguments, e.g.: def generate_multiplier(factor): def multiplier(arg, fact = factor): return arg*fact return multiplier twice = generate_multiplier(2) print twice(10) # Output: 20 ---------------------------------------------------------------------------- 4.6. How do I iterate over a sequence in reverse order? If it is a list, the fastest solution is list.reverse() try: for x in list: "do something with x" finally: list.reverse() This has the disadvantage that while you are in the loop, the list is temporarily reversed. If you don't like this, you can make a copy. This appears expensive but is actually faster than other solutions: rev = list[:] rev.reverse() for x in rev: <do something with x> If it's not a list, a more general but slower solution is: for i in range(len(sequence)-1, -1, -1): x = sequence[i] <do something with x> A more elegant solution, is to define a class which acts as a sequence and yields the elements in reverse order (solution due to Steve Majewski): class Rev: def __init__(self, seq): self.forw = seq def __len__(self): return len(self.forw) def __getitem__(self, i): return self.forw[-(i + 1)] You can now simply write: for x in Rev(list): <do something with x> Unfortunately, this solution is slowest of all, due to the method call overhead... ---------------------------------------------------------------------------- 4.7. My program is too slow. How do I speed it up? That's a tough one, in general. There are many tricks to speed up Python code; I would consider rewriting parts in C only as a last resort. One thing to notice is that function and (especially) method calls are rather expensive; if you have designed a purely OO interface with lots of tiny functions that don't do much more than get or set an instance variable or call another method, you may consider using a more direct way, e.g. directly accessing instance variables. Also see the standard module "profile" (described in the Library Reference manual) which makes it possible to find out where your program is spending most of its time (if you have some patience -- the profiling itself can slow your program down by an order of magnitude). Remember that many standard optimization heuristics you may know from other programming experience may well apply to Python. For example it may be faster to send output to output devices using larger writes rather than smaller ones in order to avoid the overhead of kernel system calls. Thus CGI scripts that write all output in "one shot" may be notably faster than those that write lots of small pieces of output. Also, be sure to use "aggregate" operations where appropriate. For example the "slicing" feature allows programs to chop up lists and other sequence objects in a single tick of the interpreter mainloop using highly optimized C implementations. Thus to get the same effect as L2 = [] for i in range[3]: L2.append(L1[i]) it is much shorter and far faster to use L2 = list(L1[:3]) # "list" is redundant if L1 is a list. Note that the map() function, particularly used with builtin methods or builtin functions can be a convenient accellerator. For example to pair the elements of two lists together: >>> map(None, [1,2,3], [4,5,6]) [(1, 4), (2, 5), (3, 6)] or to compute a number of sines: >>> map( math.sin, (1,2,3,4)) [0.841470984808, 0.909297426826, 0.14112000806, -0.756802495308] The map operation completes very quickly in such cases. Other examples of aggregate operations include the join, joinfields, split, and splitfields methods of the standard string builtin module. For example if s1..s7 are large (10K+) strings then string.joinfields([s1,s2,s3,s4,s5,s6,s7], "") may be far faster than the more obvious s1+s2+s3+s4+s5+s6+s7, since the "summation" will compute many subexpressions, whereas joinfields does all copying in one pass. For manipulating strings also consider the regular expression libraries and the "substitution" operations String % tuple and String % dictionary. Also be sure to use the list.sort builtin method to do sorting, and see FAQ's 4.51 and 4.59 for examples of moderately advanced usage -- list.sort beats other techniques for sorting in all but the most extreme circumstances. There are many other aggregate operations available in the standard libraries and in contributed libraries and extensions. Another common trick is to "push loops into functions or methods." For example suppose you have a program that runs slowly and you use the profiler (profile.run) to determine that a Python function ff is being called lots of times. If you notice that ff def ff(x): ...do something with x computing result... return result tends to be called in loops like (A) list = map(ff, oldlist) or (B) for x in sequence: value = ff(x) ...do something with value... then you can often eliminate function call overhead by rewriting ff to def ffseq(seq): resultseq = [] for x in seq: ...do something with x computing result... resultseq.append(result) return resultseq and rewrite (A) to list = ffseq(oldlist) and (B) to for value in ffseq(sequence): ...do something with value... Other single calls ff(x) translate to ffseq([x])[0] with little penalty. Of course this technique is not always appropriate and there are other variants, which you can figure out. You can gain some performance by explicitly storing the results of a function or method lookup into a local variable. A loop like for key in token: dict[key] = dict.get(key, 0) + 1 resolves dict.get every iteration. If the method isn't going to change, a faster implementation is dict_get = dict.get # look up the method once for key in token: dict[key] = dict_get(key, 0) + 1 Default arguments can be used to determine values once, at compile time instead of at run time. This can only be done for functions or objects which will not be changed during program execution, such as replacing def degree_sin(deg): return math.sin(deg * math.pi / 180.0) with def degree_sin(deg, factor = math.pi/180.0, sin = math.sin): return sin(deg * factor) Because this trick uses default arguments for terms which should not be changed, it should only be used when you are not concerned with presenting a possibly confusing API to your users. For an anecdote related to optimization, see http://www.python.org/doc/essays/list2str.html ---------------------------------------------------------------------------- 4.8. When I have imported a module, then edit it, and import it again (into the same Python process), the changes don't seem to take place. What is going on? For reasons of efficiency as well as consistency, Python only reads the module file on the first time a module is imported. (Otherwise a program consisting of many modules, each of which imports the same basic module, would read the basic module over and over again.) To force rereading of a changed module, do this: import modname reload(modname) Warning: this technique is not 100% fool-proof. In particular, modules containing statements like from modname import some_objects will continue to work with the old version of the imported objects. ---------------------------------------------------------------------------- 4.9. How do I find the current module name? A module can find out its own module name by looking at the (predefined) global variable __name__. If this has the value '__main__' you are running as a script. ---------------------------------------------------------------------------- 4.10. I have a module in which I want to execute some extra code when it is run as a script. How do I find out whether I am running as a script? See the previous question. E.g. if you put the following on the last line of your module, main() is called only when your module is running as a script: if __name__ == '__main__': main() ---------------------------------------------------------------------------- 4.11. I try to run a program from the Demo directory but it fails with ImportError: No module named ...; what gives? This is probably an optional module (written in C!) which hasn't been configured on your system. This especially happens with modules like "Tkinter", "stdwin", "gl", "Xt" or "Xm". For Tkinter, STDWIN and many other modules, see Modules/Setup.in for info on how to add these modules to your Python, if it is possible at all. Sometimes you will have to ftp and build another package first (e.g. Tcl and Tk for Tkinter). Sometimes the module only works on specific platforms (e.g. gl only works on SGI machines). NOTE: if the complaint is about "Tkinter" (upper case T) and you have already configured module "tkinter" (lower case t), the solution is not to rename tkinter to Tkinter or vice versa. There is probably something wrong with your module search path. Check out the value of sys.path. For X-related modules (Xt and Xm) you will have to do more work: they are currently not part of the standard Python distribution. You will have to ftp the Extensions tar file, i.e. ftp://ftp.python.org/pub/python/src/X-extension.tar.gz and follow the instructions there. See also the next question. ---------------------------------------------------------------------------- 4.12. I have successfully built Python with STDWIN but it can't find some modules (e.g. stdwinevents). There's a subdirectory of the library directory named 'stdwin' which should be in the default module search path. There's a line in Modules/Setup(.in) that you have to enable for this purpose -- unfortunately in the latest release it's not near the other STDWIN-related lines so it's easy to miss it. ---------------------------------------------------------------------------- 4.13. What GUI toolkits exist for Python? Depending on what platform(s) you are aiming at, there are several. Currently supported solutions: There's a neat object-oriented interface to the Tcl/Tk widget set, called Tkinter. It is part of the standard Python distribution and well-supported -- all you need to do is build and install Tcl/Tk and enable the _tkinter module and the TKPATH definition in Modules/Setup when building Python. This is probably the easiest to install and use, and the most complete widget set. It is also very likely that in the future the standard Python GUI API will be based on or at least look very much like the Tkinter interface. For more info about Tk, including pointers to the source, see the Tcl/Tk home page at http://www.scriptics.com. Tcl/Tk is now fully portable to the Mac and Windows platforms (NT and 95 only); you need Python 1.4beta3 or later and Tk 4.1patch1 or later. There's an interface to X11, including the Athena and Motif widget sets (and a few individual widgets, like Mosaic's HTML widget and SGI's GL widget) available from ftp://ftp.python.org/pub/python/src/X-extension.tar.gz. Support by Sjoerd Mullender sjoerd(a)cwi.nl. On top of the X11 interface there's the (recently revived) vpApp toolkit by Per Spilling, now also maintained by Sjoerd Mullender sjoerd(a)cwi.nl. See ftp://ftp.cwi.nl/pub/sjoerd/vpApp.tar.gz. The Mac port has a rich and ever-growing set of modules that support the native Mac toolbox calls. See the documentation that comes with the Mac port. See ftp://ftp.python.org/pub/python/mac. Support by Jack Jansen jack(a)cwi.nl. The NT port supported by Mark Hammond MHammond(a)skippinet.com.au (see question 7.2) includes an interface to the Microsoft Foundation Classes and a Python programming environment using it that's written mostly in Python. See ftp://ftp.python.org/pub/python/pythonwin/. There's an object-oriented GUI based on the Microsoft Foundation Classes model called WPY, supported by Jim Ahlstrom jim(a)interet.com. Programs written in WPY run unchanged and with native look and feel on Windows NT/95, Windows 3.1 (using win32s), and on Unix (using Tk). Source and binaries for Windows and Linux are available in ftp://ftp.python.org/pub/python/wpy/. Obsolete or minority solutions: There's an interface to wxWindows. wxWindows is a portable GUI class library written in C++. It supports XView, Motif, MS-Windows as targets. There is some support for Macs and CURSES as well. wxWindows preserves the look and feel of the underlying graphics toolkit. See the wxPython WWW page at http://www.aiai.ed.ac.uk/~jacs/wx/wxpython/wxpython.html. Support for wxPython (by Harri Pasanen pa(a)tekla.fi) appears to have a low priority. For SGI IRIX only, there are unsupported interfaces to the complete GL (Graphics Library -- low level but very good 3D capabilities) as well as to FORMS (a buttons-and-sliders-etc package built on top of GL by Mark Overmars -- ftp'able from ftp://ftp.cs.ruu.nl/pub/SGI/FORMS/). This is probably also becoming obsolete, as OpenGL takes over. There's an interface to STDWIN, a platform-independent low-level windowing interface for Mac and X11. This is totally unsupported and rapidly becoming obsolete. The STDWIN sources are at ftp://ftp.cwi.nl/pub/stdwin/. (For info about STDWIN 2.0, please refer to Steven Pemberton steven(a)cwi.nl -- I believe it is also dead.) There is an interface to WAFE, a Tcl interface to the X11 Motif and Athena widget sets. WAFE is at http://www.wu-wien.ac.at/wafe/wafe.html. (The Fresco port that was mentioned in earlier versions of this FAQ no longer seems to exist. Inquire with Mark Linton.) ---------------------------------------------------------------------------- 4.14. Are there any interfaces to database packages in Python? There's a whole collection of them in the contrib area of the ftp server, see http://www.python.org/ftp/python/contrib/Database/. ---------------------------------------------------------------------------- 4.15. Is it possible to write obfuscated one-liners in Python? Yes. See the following three examples, due to Ulf Bartelt: # Primes < 1000 print filter(None,map(lambda y:y*reduce(lambda x,y:x*y!=0, map(lambda x,y=y:y%x,range(2,int(pow(y,0.5)+1))),1),range(2,1000))) # First 10 Fibonacci numbers print map(lambda x,f=lambda x,f:(x<=1) or (f(x-1,f)+f(x-2,f)): f(x,f), range(10)) # Mandelbrot set print (lambda Ru,Ro,Iu,Io,IM,Sx,Sy:reduce(lambda x,y:x+y,map(lambda y, Iu=Iu,Io=Io,Ru=Ru,Ro=Ro,Sy=Sy,L=lambda yc,Iu=Iu,Io=Io,Ru=Ru,Ro=Ro,i=IM, Sx=Sx,Sy=Sy:reduce(lambda x,y:x+y,map(lambda x,xc=Ru,yc=yc,Ru=Ru,Ro=Ro, i=i,Sx=Sx,F=lambda xc,yc,x,y,k,f=lambda xc,yc,x,y,k,f:(k<=0)or (x*x+y*y >=4.0) or 1+f(xc,yc,x*x-y*y+xc,2.0*x*y+yc,k-1,f):f(xc,yc,x,y,k,f):chr( 64+F(Ru+x*(Ro-Ru)/Sx,yc,0,0,i)),range(Sx))):L(Iu+y*(Io-Iu)/Sy),range(Sy ))))(-2.1, 0.7, -1.2, 1.2, 30, 80, 24) # \___ ___/ \___ ___/ | | |__ lines on screen # V V | |______ columns on screen # | | |__________ maximum of "iterations" # | |_________________ range on y axis # |____________________________ range on x axis Don't try this at home, kids! ---------------------------------------------------------------------------- 4.16. Is there an equivalent of C's "?:" ternary operator? Not directly. In many cases you can mimic a?b:c with "a and b or c", but there's a flaw: if b is zero (or empty, or None -- anything that tests false) then c will be selected instead. In many cases you can prove by looking at the code that this can't happen (e.g. because b is a constant or has a type that can never be false), but in general this can be a problem. Tim Peters (who wishes it was Steve Majewski) suggested the following solution: (a and [b] or [c])[0]. Because [b] is a singleton list it is never false, so the wrong path is never taken; then applying [0] to the whole thing gets the b or c that you really wanted. Ugly, but it gets you there in the rare cases where it is really inconvenient to rewrite your code using 'if'. ---------------------------------------------------------------------------- 4.17. My class defines __del__ but it is not called when I delete the object. There are several possible reasons for this. The del statement does not necessarily call __del__ -- it simply decrements the object's reference count, and if this reaches zero __del__ is called. If your data structures contain circular links (e.g. a tree where each child has a parent pointer and each parent has a list of children) the reference counts will never go back to zero. You'll have to define an explicit close() method which removes those pointers. Please don't ever call __del__ directly -- __del__ should call close() and close() should make sure that it can be called more than once for the same object. If the object has ever been a local variable (or argument, which is really the same thing) to a function that caught an expression in an except clause, chances are that a reference to the object still exists in that function's stack frame as contained in the stack trace. Normally, deleting (better: assigning None to) sys.exc_traceback will take care of this. If a stack was printed for an unhandled exception in an interactive interpreter, delete sys.last_traceback instead. There is code that deletes all objects when the interpreter exits, but it is not called if your Python has been configured to support threads (because other threads may still be active). You can define your own cleanup function using sys.exitfunc (see question 4.4). Finally, if your __del__ method raises an exception, this will be ignored. Starting with Python 1.4beta3, a warning message is printed to sys.stderr when this happens. See also question 6.14 for a discussion of the possibility of adding true garbage collection to Python. ---------------------------------------------------------------------------- 4.18. How do I change the shell environment for programs called using os.popen() or os.system()? Changing os.environ doesn't work. You must be using either a version of python before 1.4, or on a (rare) system that doesn't have the putenv() library function. Before Python 1.4, modifying the environment passed to subshells was left out of the interpreter because there seemed to be no well-established portable way to do it (in particular, some systems, have putenv(), others have setenv(), and some have none at all). As of Python 1.4, almost all Unix systems do have putenv(), and so does the Win32 API, and thus the os module was modified so that changes to os.environ are trapped and the corresponding putenv() call is made. ---------------------------------------------------------------------------- 4.19. What is a class? A class is the particular object type that is created by executing a class statement. Class objects are used as templates, to create class instance objects, which embody both the data structure and program routines specific to a datatype. ---------------------------------------------------------------------------- 4.20. What is a method? A method is a function that you normally call as x.name(arguments...) for some object x. The term is used for methods of classes and class instances as well as for methods of built-in objects. (The latter have a completely different implementation and only share the way their calls look in Python code.) Methods of classes (and class instances) are defined as functions inside the class definition. ---------------------------------------------------------------------------- 4.21. What is self? Self is merely a conventional name for the first argument of a method -- i.e. a function defined inside a class definition. A method defined as meth(self, a, b, c) should be called as x.meth(a, b, c) for some instance x of the class in which the definition occurs; the called method will think it is called as meth(x, a, b, c). ---------------------------------------------------------------------------- 4.22. What is an unbound method? An unbound method is a method defined in a class that is not yet bound to an instance. You get an unbound method if you ask for a class attribute that happens to be a function. You get a bound method if you ask for an instance attribute. A bound method knows which instance it belongs to and calling it supplies the instance automatically; an unbound method only knows which class it wants for its first argument (a derived class is also OK). Calling an unbound method doesn't "magically" derive the first argument from the context -- you have to provide it explicitly. Trivia note regarding bound methods: each reference to a bound method of a particular object creates a bound method object. If you have two such references (a = inst.meth; b = inst.meth), they will compare equal (a == b) but are not the same (a is not b). ---------------------------------------------------------------------------- 4.23. How do I call a method defined in a base class from a derived class that overrides it? If your class definition starts with "class Derived(Base): ..." then you can call method meth defined in Base (or one of Base's base classes) as Base.meth(self, arguments...). Here, Base.meth is an unbound method (see previous question). ---------------------------------------------------------------------------- 4.24. How do I call a method from a base class without using the name of the base class? DON'T DO THIS. REALLY. I MEAN IT. It appears that you could call self.__class__.__bases__[0].meth(self, arguments...) but this fails when a doubly-derived method is derived from your class: for its instances, self.__class__.__bases__[0] is your class, not its base class -- so (assuming you are doing this from within Derived.meth) you would start a recursive call. Often when you want to do this you are forgetting that classes are first class in Python. You can "point to" the class you want to delegate an operation to either at the instance or at the subclass level. For example if you want to use a "glorp" operation of a superclass you can point to the right superclass to use. class subclass(superclass1, superclass2, superclass3): delegate_glorp = superclass2 ... def glorp(self, arg1, arg2): ... subclass specific stuff ... self.delegate_glorp.glorp(self, arg1, arg2) ... class subsubclass(subclass): delegate_glorp = superclass3 ... Note, however that setting delegate_glorp to subclass in subsubclass would cause an infinite recursion on subclass.delegate_glorp. Careful! Maybe you are getting too fancy for your own good. Consider simplifying the design (?). ---------------------------------------------------------------------------- 4.25. How can I organize my code to make it easier to change the base class? You could define an alias for the base class, assign the real base class to it before your class definition, and use the alias throughout your class. Then all you have to change is the value assigned to the alias. Incidentally, this trick is also handy if you want to decide dynamically (e.g. depending on availability of resources) which base class to use. Example: BaseAlias = <real base class> class Derived(BaseAlias): def meth(self): BaseAlias.meth(self) ... ---------------------------------------------------------------------------- 4.26. How can I find the methods or attributes of an object? This depends on the object type. For an instance x of a user-defined class, instance attributes are found in the dictionary x.__dict__, and methods and attributes defined by its class are found in x.__class__.__bases__[i].__dict__ (for i in range(len(x.__class__.__bases__))). You'll have to walk the tree of base classes to find all class methods and attributes. Many, but not all built-in types define a list of their method names in x.__methods__, and if they have data attributes, their names may be found in x.__members__. However this is only a convention. For more information, read the source of the standard (but undocumented) module newdir. ---------------------------------------------------------------------------- 4.27. I can't seem to use os.read() on a pipe created with os.popen(). os.read() is a low-level function which takes a file descriptor (a small integer). os.popen() creates a high-level file object -- the same type used for sys.std{in,out,err} and returned by the builtin open() function. Thus, to read n bytes from a pipe p created with os.popen(), you need to use p.read(n). ---------------------------------------------------------------------------- 4.28. How can I create a stand-alone binary from a Python script? The "freeze" tool in "Tools/freeze/" does what you want. See the README. This works by scanning your source recursively for import statements (both forms) and looking for the modules on the standard Python path as well as in the source directory (for built-in modules). It then "compiles" the modules written in Python to C code (array initializers that can be turned into code objects using the marshal module) and creates a custom-made config file that only contains those built-in modules which are actually used in the program. It then compiles the generated C code and links it with the rest of the Python interpreter to form a self-contained binary which acts exactly like your script. Hint: the freeze program only works if your script's filename ends in ".py". ---------------------------------------------------------------------------- 4.29. What WWW tools are there for Python? See the chapter titled "Internet and WWW" in the Library Reference Manual. There's also a web browser written in Python, called Grail -- see http://grail.cnri.reston.va.us/grail/. ---------------------------------------------------------------------------- 4.30. How do I run a subprocess with pipes connected to both input and output? Use the standard popen2 module. For example: import popen2 fromchild, tochild = popen2.popen2("command") tochild.write("input\n") tochild.flush() output = fromchild.readline() Warning: in general, it is unwise to do this, because you can easily cause a deadlock where your process is blocked waiting for output from the child, while the child is blocked waiting for input from you. This can be caused because the parent expects the child to output more text than it does, or it can be caused by data being stuck in stdio buffers due to lack of flushing. The Python parent can of course explicitly flush the data it sends to the child before it reads any output, but if the child is a naive C program it can easily have been written to never explicitly flush its output, even if it is interactive, since flushing is normally automatic. Note on a bug in popen2: unless your program calls wait() or waitpid(), finished child processes are never removed, and eventually calls to popen2 will fail because of a limit on the number of child processes. Calling os.waitpid with the os.WNOHANG option can prevent this; a good place to insert such a call would be before calling popen2 again. In many cases, all you really need is to run some data through a command and get the result back. Unless the data is infinite in size, the easiest (and often the most efficient!) way to do this is to write it to a temporary file and run the command with that temporary file as input. The standard module tempfile exports a function mktemp() which generates unique temporary file names. Note that many interactive programs (e.g. vi) don't work well with pipes substituted for standard input and output. You will have to use pseudo ttys ("ptys") instead of pipes. There is some undocumented code to use these in the library module pty.py -- I'm afraid you're on your own here. A different answer is a Python interface to Don Libes' "expect" library. A Python extension that interfaces to expect is called "expy" and available from ftp://ftp.python.org/pub/python/contrib/System/. A pure Python solution that works like expect is PIPE by John Croix. A prerelease of PIPE is available from ftp://ftp.python.org/pub/python/contrib/System/. ---------------------------------------------------------------------------- 4.31. How do I call a function if I have the arguments in a tuple? Use the built-in function apply(). For instance, func(1, 2, 3) is equivalent to args = (1, 2, 3) apply(func, args) Note that func(args) is not the same -- it calls func() with exactly one argument, the tuple args, instead of three arguments, the integers 1, 2 and 3. ---------------------------------------------------------------------------- 4.32. How do I enable font-lock-mode for Python in Emacs? If you are using XEmacs 19.14 or later, any XEmacs 20, FSF Emacs 19.34 or any Emacs 20, font-lock should work automatically for you if you are using the latest python-mode.el. If you are using an older version of XEmacs or Emacs you will need to put this in your .emacs file: (defun my-python-mode-hook () (setq font-lock-keywords python-font-lock-keywords) (font-lock-mode 1)) (add-hook 'python-mode-hook 'my-python-mode-hook) ---------------------------------------------------------------------------- 4.33. Is there a scanf() or sscanf() equivalent? Not as such. For simple input parsing, the easiest approach is usually to split the line into whitespace-delimited words using string.split(), and to convert decimal strings to numeric values using string.atoi(), string.atol() or string.atof(). (Python's atoi() is 32-bit and its atol() is arbitrary precision.) If you want to use another delimiter than whitespace, use string.splitfield() (possibly combining it with string.strip() which removes surrounding whitespace from a string). For more complicated input parsing, regular expressions (see module regex) are better suited and more powerful than C's sscanf(). There's a contributed module that emulates sscanf(), by Steve Clift; see contrib/Misc/sscanfmodule.c of the ftp site: http://www.python.org/ftp/python/contrib/Misc/sscanfmodule.c ---------------------------------------------------------------------------- 4.34. Can I have Tk events handled while waiting for I/O? Yes, and you don't even need threads! But you'll have to restructure your I/O code a bit. Tk has the equivalent of Xt's XtAddInput() call, which allows you to register a callback function which will be called from the Tk mainloop when I/O is possible on a file descriptor. Here's what you need: from Tkinter import tkinter tkinter.createfilehandler(file, mask, callback) The file may be a Python file or socket object (actually, anything with a fileno() method), or an integer file descriptor. The mask is one of the constants tkinter.READABLE or tkinter.WRITABLE. The callback is called as follows: callback(file, mask) You must unregister the callback when you're done, using tkinter.deletefilehandler(file) Note: since you don't know *how many bytes* are available for reading, you can't use the Python file object's read or readline methods, since these will insist on reading a predefined number of bytes. For sockets, the recv() or recvfrom() methods will work fine; for other files, use os.read(file.fileno(), maxbytecount). ---------------------------------------------------------------------------- 4.35. How do I write a function with output parameters (call by reference)? [Mark Lutz] The thing to remember is that arguments are passed by assignment in Python. Since assignment just creates references to objects, there's no alias between an argument name in the caller and callee, and so no call-by-reference per se. But you can simulate it in a number of ways: 1) By using global variables; but you probably shouldn't :-) 2) By passing a mutable (changeable in-place) object: def func1(a): a[0] = 'new-value' # 'a' references a mutable list a[1] = a[1] + 1 # changes a shared object args = ['old-value', 99] func1(args) print args[0], args[1] # output: new-value 100 3) By returning a tuple, holding the final values of arguments: def func2(a, b): a = 'new-value' # a and b are local names b = b + 1 # assigned to new objects return a, b # return new values x, y = 'old-value', 99 x, y = func2(x, y) print x, y # output: new-value 100 4) And other ideas that fall-out from Python's object model. For instance, it might be clearer to pass in a mutable dictionary: def func3(args): args['a'] = 'new-value' # args is a mutable dictionary args['b'] = args['b'] + 1 # change it in-place args = {'a':' old-value', 'b': 99} func3(args) print args['a'], args['b'] 5) Or bundle-up values in a class instance: class callByRef: def __init__(self, **args): for (key, value) in args.items(): setattr(self, key, value) def func4(args): args.a = 'new-value' # args is a mutable callByRef args.b = args.b + 1 # change object in-place args = callByRef(a='old-value', b=99) func4(args) print args.a, args.b But there's probably no good reason to get this complicated :-). [Python's author favors solution 3 in most cases.] ---------------------------------------------------------------------------- 4.36. Please explain the rules for local and global variables in Python. [Ken Manheimer] In Python, procedure variables are implicitly global, unless they are assigned anywhere within the block. In that case they are implicitly local, and you need to explicitly declare them as 'global'. Though a bit surprising at first, a moment's consideration explains this. On one hand, requirement of 'global' for assigned vars provides a bar against unintended side-effects. On the other hand, if global were required for all global references, you'd be using global all the time. Eg, you'd have to declare as global every reference to a builtin function, or to a component of an imported module. This clutter would defeat the usefulness of the 'global' declaration for identifying side-effects. ---------------------------------------------------------------------------- 4.37. How can I have modules that mutually import each other? Jim Roskind recommends the following order in each module: First: all exports (like globals, functions, and classes that don't need imported base classes). Then: all import statements. Finally: all active code (including globals that are initialized from imported values). Python's author doesn't like this approach much because the imports appear in a strange place, but has to admit that it works. His recommended strategy is to avoid all uses of "from <module> import *" (so everything from an imported module is referenced as <module>.<name>) and to place all code inside functions. Initializations of global variables and class variables should use constants or built-in functions only. ---------------------------------------------------------------------------- 4.38. How do I copy an object in Python? There is no generic copying operation built into Python, however most object types have some way to create a clone. Here's how for the most common objects: For immutable objects (numbers, strings, tuples), cloning is unnecessary since their value can't change. For lists (and generally for mutable sequence types), a clone is created by the expression l[:]. For dictionaries, the following function returns a clone: def dictclone(o): n = {} for k in o.keys(): n[k] = o[k] return n Finally, for generic objects, the "copy" module defines two functions for copying objects. copy.copy(x) returns a copy as shown by the above rules. copy.deepcopy(x) also copies the elements of composite objects. See the section on this module in the Library Reference Manual. ---------------------------------------------------------------------------- 4.39. How to implement persistent objects in Python? (Persistent == automatically saved to and restored from disk.) The library module "pickle" now solves this in a very general way (though you still can't store things like open files, sockets or windows), and the library module "shelve" uses pickle and (g)dbm to create persistent mappings containing arbitrary Python objects. For possibly better performance also look for the latest version of the relatively recent cPickle module. A more awkward way of doing things is to use pickle's little sister, marshal. The marshal module provides very fast ways to store noncircular basic Python types to files and strings, and back again. Although marshal does not do fancy things like store instances or handle shared references properly, it does run extremely fast. For example loading a half megabyte of data may take less than a third of a second (on some machines). This often beats doing something more complex and general such as using gdbm with pickle/shelve. ---------------------------------------------------------------------------- 4.40. I try to use __spam and I get an error about _SomeClassName__spam. Variables with double leading underscore are "mangled" to provide a simple but effective way to define class private variables. See the chapter "New in Release 1.4" in the Python Tutorial. ---------------------------------------------------------------------------- 4.41. How do I delete a file? And other file questions. Use os.remove(filename) or os.unlink(filename); for documentation, see the posix section of the library manual. They are the same, unlink() is simply the Unix name for this function. In earlier versions of Python, only os.unlink() was available. To remove a directory, use os.rmdir(); use os.mkdir() to create one. To rename a file, use os.rename(). To truncate a file, open it using f = open(filename, "r+"), and use f.truncate(offset); offset defaults to the current seek position. (The "r+" mode opens the file for reading and writing.) There's also os.ftruncate(fd, offset) for files opened with os.open() -- for advanced Unix hacks only. ---------------------------------------------------------------------------- 4.42. How to modify urllib or httplib to support HTTP/1.1? Apply the following patch to the vanilla Python 1.4 httplib.py: 41c41 < replypat = regsub.gsub('\\.', '\\\\.', HTTP_VERSION) + \ --- > replypat = regsub.gsub('\\.', '\\\\.', 'HTTP/1.[0-9]+') + \ ---------------------------------------------------------------------------- 4.43. Unexplicable syntax errors in compile() or exec. When a statement suite (as opposed to an expression) is compiled by compile(), exec or execfile(), it must end in a newline. In some cases, when the source ends in an indented block it appears that at least two newlines are required. ---------------------------------------------------------------------------- 4.44. How do I convert a string to a number? For integers, use the built-in int() function, e.g. int('144') == 144. Similarly, long() converts from string to long integer, e.g. long('144') == 144L; and float() to floating-point, e.g. float('144') == 144.0. Note that these are restricted to decimal interpretation, so that int('0144') == 144 and int('0x144') raises ValueError. For greater flexibility, or before Python 1.5, import the module string and use the string.atoi() function for integers, string.atol() for long integers, or string.atof() for floating-point. E.g., string.atoi('100', 16) == string.atoi('0x100', 0) == 256. See the library reference manual section for the string module for more details. While you could use the built-in function eval() instead of any of those, this is not recommended, because someone could pass you a Python expression that might have unwanted side effects (like reformatting your disk). ---------------------------------------------------------------------------- 4.45. How do I convert a number to a string? To convert, e.g., the number 144 to the string '144', use the built-in function repr() or the backquote notation (these are equivalent). If you want a hexadecimal or octal representation, use the built-in functions hex() or oct(), respectively. For fancy formatting, use the % operator on strings, just like C printf formats, e.g. "%04d" % 144 yields '0144' and "%.3f" % (1/3.0) yields '0.333'. See the library reference manual for details. ---------------------------------------------------------------------------- 4.46. How do I copy a file? Most of the time this will do: infile = open("file.in", "rb") outfile = open("file.out", "wb") outfile.write(infile.read()) However for huge files you may want to do the reads/writes in pieces (or you may have to), and if you dig deeper you may find other technical problems. Unfortunately, there's no totally platform independent answer. On Unix, you can use os.system() to invoke the "cp" command (see your Unix manual for how it's invoked). On DOS or Windows, use os.system() to invoke the "COPY" command. On the Mac, use macostools.copy(srcpath, dstpath). It will also copy the resource fork and Finder info. There's also the shutil module which contains a copyfile() function that implements the copy loop; but in Python 1.4 and earlier it opens files in text mode, and even in Python 1.5 it still isn't good enough for the Macintosh: it doesn't copy the resource fork and Finder info. ---------------------------------------------------------------------------- 4.47. How do I check if an object is an instance of a given class or of a subclass of it? If you are developing the classes from scratch it might be better to program in a more proper object-oriented style -- instead of doing a different thing based on class membership, why not use a method and define the method differently in different classes? However, there are some legitimate situations where you need to test for class membership. In Python 1.5, you can use the built-in function isinstance(obj, cls). The following approaches can be used with earlier Python versions: An unobvious method is to raise the object as an exception and to try to catch the exception with the class you're testing for: def is_instance_of(the_instance, the_class): try: raise the_instance except the_class: return 1 except: return 0 This technique can be used to distinguish "subclassness" from a collection of classes as well try: raise the_instance except Audible: the_instance.play(largo) except Visual: the_instance.display(gaudy) except Olfactory: sniff(the_instance) except: raise ValueError, "dunno what to do with this!" This uses the fact that exception catching tests for class or subclass membership. A different approach is to test for the presence of a class attribute that is presumably unique for the given class. For instance: class MyClass: ThisIsMyClass = 1 ... def is_a_MyClass(the_instance): return hasattr(the_instance, 'ThisIsMyClass') This version is easier to inline, and probably faster (inlined it is definitely faster). The disadvantage is that someone else could cheat: class IntruderClass: ThisIsMyClass = 1 # Masquerade as MyClass ... but this may be seen as a feature (anyway, there are plenty of other ways to cheat in Python). Another disadvantage is that the class must be prepared for the membership test. If you do not "control the source code" for the class it may not be advisable to modify the class to support testability. ---------------------------------------------------------------------------- 4.48. What is delegation? Delegation refers to an object oriented technique Python programmers may implement with particular ease. Consider the following: from string import upper class UpperOut: def __init__(self, outfile): self.__outfile = outfile def write(self, str): self.__outfile.write( upper(str) ) def __getattr__(self, name): return getattr(self.__outfile, name) Here the UpperOut class redefines the write method to convert the argument string to upper case before calling the underlying self.__outfile.write method, but all other methods are delegated to the underlying self.__outfile object. The delegation is accomplished via the "magic" __getattr__ method. Please see the language reference for more information on the use of this method. Note that for more general cases delegation can get trickier. Particularly when attributes must be set as well as gotten the class must define a __settattr__ method too, and it must do so carefully. The basic implementation of __setattr__ is roughly equivalent to the following: class X: ... def __setattr__(self, name, value): self.__dict__[name] = value ... Most __setattr__ implementations must modify self.__dict__ to store local state for self without causing an infinite recursion. ---------------------------------------------------------------------------- 4.49. How do I test a Python program or component. First, it helps to write the program so that it may be easily tested by using good modular design. In particular your program should have almost all functionality encapsulated in either functions or class methods -- and this sometimes has the surprising and delightful effect of making the program run faster (because local variable accesses are faster than global accesses). Furthermore the program should avoid depending on mutating global variables, since this makes testing much more difficult to do. The "global main logic" of your program may be as simple as if __name__=="__main__": main_logic() at the bottom of the main module of your program. Once your program is organized as a tractible collection of functions and class behaviours you should write test functions that exercise the behaviours. A test suite can be associated with each module which automates a sequence of tests. This sounds like a lot of work, but since Python is so terse and flexible it's surprisingly easy. You can make coding much more pleasant and fun by writing your test functions in parallel with the "production code", since this makes it easy to find bugs and even design flaws earlier. "Support modules" that are not intended to be the main module of a program may include a "test script interpretation" which invokes a self test of the module. if __name__ == "__main__": self_test() Even programs that interact with complex external interfaces may be tested when the external interfaces are unavailable by using "fake" interfaces implemented in Python. For an example of a "fake" interface, the following class defines (part of) a "fake" file interface: import string testdata = "just a random sequence of characters" class FakeInputFile: data = testdata position = 0 closed = 0 def read(self, n=None): self.testclosed() p = self.position if n is None: result= self.data[p:] else: result= self.data[p: p+n] self.position = p + len(result) return result def seek(self, n, m=0): self.testclosed() last = len(self.data) p = self.position if m==0: final=n elif m==1: final=n+p elif m==2: final=len(self.data)+n else: raise ValueError, "bad m" if final<0: raise IOError, "negative seek" self.position = final def isatty(self): return 0 def tell(self): return self.position def close(self): self.closed = 1 def testclosed(self): if self.closed: raise IOError, "file closed" Try f=FakeInputFile() and test out its operations. ---------------------------------------------------------------------------- 4.50. My multidimensional list (array) is broken! What gives? You probably tried to make a multidimensional array like this. A = [[None] * 2] * 3 This makes a list containing 3 references to the same list of length two. Changes to one row will show in all rows, which is probably not what you want. The following works much better: A = [None]*3 for i in range(3): A[i] = [None] * 2 This generates a list containing 3 different lists of length two. If you feel weird, you can also do it in the following way: w, h = 2, 3 A = map(lambda i,w=w: [None] * w, range(h)) ---------------------------------------------------------------------------- 4.51. I want to do a complicated sort: can you do a Schwartzian Transform in Python? Yes, and in Python you only have to write it once: def st(List, Metric): def pairing(element, M = Metric): return (M(element), element) paired = map(pairing, List) paired.sort() return map(stripit, paired) def stripit(pair): return pair[1] This technique, attributed to Randal Schwartz, sorts the elements of a list by a metric which maps each element to its "sort value". For example, if L is a list of string then import string Usorted = st(L, string.upper) def intfield(s): return string.atoi( string.strip(s[10:15] ) ) Isorted = st(L, intfield) Usorted gives the elements of L sorted as if they were upper case, and Isorted gives the elements of L sorted by the integer values that appear in the string slices starting at position 10 and ending at position 15. Note that Isorted may also be computed by def Icmp(s1, s2): return cmp( intfield(s1), intfield(s2) ) Isorted = L[:] Isorted.sort(Icmp) but since this method computes intfield many times for each element of L, it is slower than the Schwartzian Transform. ---------------------------------------------------------------------------- 4.52. How to convert between tuples and lists? The function tuple(seq) converts any sequence into a tuple with the same items in the same order. For example, tuple([1, 2, 3]) yields (1, 2, 3) and tuple('abc') yields ('a', 'b', 'c'). If the argument is a tuple, it does not make a copy but returns the same object, so it is cheap to call tuple() when you aren't sure that an object is already a tuple. The function list(seq) converts any sequence into a list with the same items in the same order. For example, list((1, 2, 3)) yields [1, 2, 3] and list('abc') yields ['a', 'b', 'c']. If the argument is a list, it makes a copy just like seq[:] would. ---------------------------------------------------------------------------- 4.53. Files retrieved with urllib contain leading garbage that looks like email headers. The server is using HTTP/1.1; the vanilla httplib in Python 1.4 only recognizes HTTP/1.0. See question 4.42 for a patch. ---------------------------------------------------------------------------- 4.54. How do I get a list of all instances of a given class? Python does not keep track of all instances of a class (or of a built-in type). You can program the class's constructor to keep track of all instances, but unless you're very clever, this has the disadvantage that the instances never get deleted,because your list of all instances keeps a reference to them. (The trick is to regularly inspect the reference counts of the instances you've retained, and if the reference count is below a certain level, remove it from the list. Determining that level is tricky -- it's definitely larger than 1.) ---------------------------------------------------------------------------- 4.55. A regular expression fails with regex.error: match failure. This is usually caused by too much backtracking; the regular expression engine has a fixed size stack which holds at most 4000 backtrack points. Every character matched by e.g. ".*" accounts for a backtrack point, so even a simple search like regex.match('.*x',"x"*5000) will fail. This is fixed in the re module introduced with Python 1.5; consult the Library Reference section on re for more information. ---------------------------------------------------------------------------- 4.56. I can't get signal handlers to work. The most common problem is that the signal handler is declared with the wrong argument list. It is called as handler(signum, frame) so it should be declared with two arguments: def handler(signum, frame): ... ---------------------------------------------------------------------------- 4.57. I can't use a global variable in a function? Help! Did you do something like this? x = 1 # make a global def f(): print x # try to print the global ... for j in range(100): if q>3: x=4 If you did, all references to x in f are local, not global by virtue of the "x=4" assignment. Any variable assigned in a function is local to that function unless it is declared global. Consequently the "print x" attempts to print an uninitialized local variable and will trigger a NameError. ---------------------------------------------------------------------------- 4.58. What's a negative index? Why doesn't list.insert() use them? Python sequences are indexed with positive numbers and negative numbers. For positive numbers 0 is the first index 1 is the second index and so forth. For negative indices -1 is the last index and -2 is the pentultimate (next to last) index and so forth. Think of seq[-n] as the same as seq[len(seq)-n]. Using negative indices can be very convenient. For example if the string Line ends in a newline then Line[:-1] is all of Line except the newline. Sadly the list builtin method L.insert does not observe negative indices. This feature could be considered a mistake but since existing programs depend on this feature it may stay around forever. L.insert for negative indices inserts at the start of the list. To get "proper" negative index behaviour use L[n:n] = [x] in place of the insert method. ---------------------------------------------------------------------------- 4.59. How can I sort one list by values from another list? You can sort lists of tuples. >>> list1 = ["what", "I'm", "sorting", "by"] >>> list2 = ["something", "else", "to", "sort"] >>> pairs = map(None, list1, list2) >>> pairs [('what', 'something'), ("I'm", 'else'), ('sorting', 'to'), ('by', 'sort')] >>> pairs.sort() >>> pairs [("I'm", 'else'), ('by', 'sort'), ('sorting', 'to'), ('what', 'something')] >>> result = pairs[:] >>> for i in xrange(len(result)): result[i] = result[i][1] ... >>> result ['else', 'sort', 'to', 'something'] And if you didn't understand the question, please see the example above ;c). Note that "I'm" sorts before "by" because uppercase "I" comes before lowercase "b" in the ascii order. Also see 4.51. ---------------------------------------------------------------------------- 4.60. Why doesn't dir() work on builtin types like files and lists? It should have -- and it does starting with Python 1.5 (currently in development -- see Questions 1.13 and 2.10). Using 1.4, you can find out which methods a given object supports by looking at its __methods__ attribute: >>> List = [] >>> List.__methods__ ['append', 'count', 'index', 'insert', 'remove', 'reverse', 'sort'] ---------------------------------------------------------------------------- 4.61. How can I mimic CGI form submission (METHOD=POST)? I would like to retrieve web pages that are the result of POSTing a form. Is there existing code that would let me do this easily? Yes. Here's a simple example that uses httplib. #!/usr/local/bin/python import httplib, sys, time ### build the query string qs = "First=Josephine&MI=Q&Last=Public" ### connect and send the server a path httpobj = httplib.HTTP('www.some-server.out-there', 80) httpobj.putrequest('POST', '/cgi-bin/some-cgi-script') ### now generate the rest of the HTTP headers... httpobj.putheader('Accept', '*/*') httpobj.putheader('Connection', 'Keep-Alive') httpobj.putheader('Content-type', 'application/x-www-form-urlencoded') httpobj.putheader('Content-length', '%d' % len(qs)) httpobj.endheaders() httpobj.send(qs) ### find out what the server said in response... reply, msg, hdrs = httpobj.getreply() if reply != 200: sys.stdout.write(httpobj.getfile().read()) Note that in general for "url encoded posts" (the default) query strings must be "quoted" to, for example, change equals signs and spaces to an encoded form when they occur in name or value. Use urllib.quote to perform this quoting. For example to send name="Guy Steele, Jr.": >>> from urllib import quote >>> x = quote("Guy Steele, Jr.") >>> x 'Guy%20Steele,%20Jr.' >>> query_string = "name="+x >>> query_string 'name=Guy%20Steele,%20Jr.' ---------------------------------------------------------------------------- 4.62. If my program crashes with a bsddb (or anydbm) database open, it gets corrupted. How come? Databases opened for write access with the bsddb module (and often by the anydbm module, since it will preferentially use bsddb) must explcitly be closed using the close method of the database. The underlying libdb package caches database contents which need to be converted to on-disk form and written, unlike regular open files which already have the on-disk bits in the kernel's write buffer, where they can just be dumped by the kernel with the program exits. If you have initialized a new bsddb database but not written anything to it before the program crashes, you will often wind up with a zero-length file and encounter an exception the next time the file is opened. ---------------------------------------------------------------------------- 4.63. How do I make a Python script executable on Unix? You need to do two things: the script file's mode must be executable (include the 'x' bit), and the first line must begin with #! followed by the pathname for the Python interpreter. The first is done by executing 'chmod +x scriptfile' or perhaps 'chmod 755 scriptfile'. The second can be done in a number of way. The most straightforward way is to write #!/usr/local/bin/python as the very first ine of your file - or whatever the pathname is where the python interpreter is installed on your platform. If you would like the script to be independent of where the python interpreter lives, you can use the "env" program. On almost all platforms, the following woll work, assuming the python interpreter is in a directory on the user's $PATH: #! /usr/bin/env python Note -- *don't* do this for CGI scripts. The $PATH variable for CGI scripts is often very minimal, so you need to use the actual absolute pathname of the interpreter. Occasionally, a user's environment is so full that the /usr/bin/env program fails; or there's no env program at all. In that case, you can try the following hack (due to Alex Rezinsky): #! /bin/sh """:" exec python $0 ${1+"$@"} """ The disadvantage is that this defines the script's __doc__ string. However, you can fix that by adding __doc__ = """...Whatever...""" ---------------------------------------------------------------------------- 4.64. How do you remove duplicates from a list? Generally, if you don't mind reordering the List if List: List.sort() last = List[-1] for i in range(len(List)-2, -1, -1): if last==List[i]: del List[i] else: last=List[i] If all elements of the list may be used as dictionary keys (ie, they are all hashable) this is often faster d = {} for x in List: d[x]=x List = d.values() Also, for extremely large lists you might consider more optimal alternatives to the first one. The second one is pretty good whenever it can be used. ---------------------------------------------------------------------------- 4.65. Are there any known year 2000 problems in Python? I am not aware of year 2000 deficiencies in Python 1.5. Python does very few date calculations and for what it does, it relies on the C library functions. Python generally represent times either as seconds since 1970 or as a tuple (year, month, day, ...) where the year is expressed with four digits, which makes Y2K bugs unlikely. So as long as your C library is okay, Python should be okay. Of course, I cannot vouch for your Python code! Given the nature of freely available software, I have to add that this statement is not legally binding. The Python copyright notice contains the following disclaimer: STICHTING MATHEMATISCH CENTRUM AND CNRI DISCLAIM ALL WARRANTIES WITH REGARD TO THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO EVENT SHALL STICHTING MATHEMATISCH CENTRUM OR CNRI BE LIABLE FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. The good news is that if you encounter a problem, you have full source available to track it down and fix it! ---------------------------------------------------------------------------- 4.66. I want a version of map that applies a method to a sequence of objects! Help! Get fancy! def method_map(objects, method, arguments): """method_map([a,b], "flog", (1,2)) gives [a.flog(1,2), b.flog(1,2)]""" nobjects = len(objects) methods = map(getattr, objects, [method]*nobjects) return map(apply, methods, [arguments]*nobjects) It's generally a good idea to get to know the mysteries of map and apply and getattr and the other dynamic features of Python. ---------------------------------------------------------------------------- 4.67. How do I generate random numbers in Python? The standard library module "whrandom" implements a random number generator. Usage is simple: import whrandom whrandom.random() This returns a random floating point number in the range [0, 1). There are also other specialized generators in this module: randint(a, b) chooses an integer in the range [a, b) choice(S) chooses from a given sequence uniform(a, b) chooses a floating point number in the range [a, b) To force the random number generator's initial setting, use seed(x, y, z) set the seed from three integers in [1, 256) There's also a class, whrandom, whoch you can instantiate to create independent multiple random number generators. The module "random" contains functions that approximate various standard distributions. All this is documented in the library reference manual. Note that the module "rand" is obsolete. ---------------------------------------------------------------------------- 4.68. How do I access the serial (RS232) port? There's a Windows serial communication module (for communication over RS 232 serial ports) at http://www.python.org/ftp/python/contrib/System/siomodule.README http://www.python.org/ftp/python/contrib/System/siomodule.zip For DOS, try Hans Nowak's Python-DX, which supports this, at: http://www.cuci.nl/~hnowak/ For Unix, search Deja News (using http://www.python.org/search/) for "serial port" with author Mitch Chapman (his post is a little too long to include here). ---------------------------------------------------------------------------- 4.69. Images on Tk-Buttons don't work in Py15? They do work, but you must keep your own reference to the image object now. More verbosely, you must make sure that, say, a global variable or a class attribute refers to the object. Quoting Fredrik Lundh from the mailinglist: Well, the Tk button widget keeps a reference to the internal photoimage object, but Tkinter does not. So when the last Python reference goes away, Tkinter tells Tk to release the photoimage. But since the image is in use by a widget, Tk doesn't destroy it. Not completely. It just blanks the image, making it completely transparent... And yes, there was a bug in the keyword argument handling in 1.4 that kept an extra reference around in some cases. And when Guido fixed that bug in 1.5, he broke quite a few Tkinter programs... ---------------------------------------------------------------------------- 4.70. Where is the math.py (socket.py, regex.py, etc.) source file? If you can't find a source file for a module it may be a builtin or dynamically loaded module implemented in C, C++ or other compiled language. In this case you may not have the source file or it may be something like mathmodule.c, somewhere in a C source directory (not on the Python Path). Fredrik Lundh (fredrik(a)pythonware.com) explains (on the python-list): There are (at least) three kinds of modules in Python: 1) modules written in Python (.py); 2) modules written in C and dynamically loaded (.dll, .pyd, .so, .sl, etc); 3) modules written in C and linked with the interpreter; to get a list of these, type: import sys print sys.builtin_module_names ---------------------------------------------------------------------------- 4.71. How do I send mail from a Python script? On Unix, it's very simple, using sendmail. The location of the sendmail program varies between systems; sometimes it is /usr/lib/sendmail, sometime /usr/sbin/sendmail. The sendmail manual page will help you out. Here's some sample code: SENDMAIL = "/usr/sbin/sendmail" # sendmail location import os p = os.popen("%s -t" % SENDMAIL, "w") p.write("To: cary(a)ratatosk.org\n") p.write("Subject: test\n") p.write("\n") # blank line separating headers from body p.write("Some text\n") p.write("some more text\n") sts = p.close() if sts != 0: print "Sendmail exit status", sts On non-Unix systems (and on Unix systems too, of course!), you can use SMTP to send mail to a nearby mail server. A library for SMTP (smtplib.py) is included in Python 1.5.1; in 1.5.2 it will be documented and extended. Here's a very simple interactive mail sender that uses it: import sys, smtplib fromaddr = raw_input("From: ") toaddrs = string.splitfields(raw_input("To: "), ',') print "Enter message, end with ^D:" msg = '' while 1: line = sys.stdin.readline() if not line: break msg = msg + line # The actual mail send server = smtplib.SMTP('localhost') server.sendmail(fromaddr, toaddrs, msg) server.quit() This method will work on any host that supports an SMTP listener; otherwise, you will have to ask the user for a host. ---------------------------------------------------------------------------- 4.72. How do I avoid blocking in connect() of a socket? The select module is widely known to help with asynchronous I/O on sockets once they are connected. However, it is less than common knowledge how to avoid blocking on the initial connect() call. Jeremy Hylton has the following advice (slightly edited): To prevent the TCP connect from blocking, you can set the socket to non-blocking mode. Then when you do the connect(), you will either connect immediately (unlikely) or get an exception that contains the errno. errno.EINPROGRESS indicates that the connection is in progress, but hasn't finished yet. Different OSes will return different errnos, so you're going to have to check. I can tell you that different versions of Solaris return different errno values. In Python 1.5 and later, you can use connect_ex() to avoid creating an exception. It will just return the errno value. To poll, you can call connect_ex() again later -- 0 or errno.EISCONN indicate that you're connected -- or you can pass this socket to select (checking to see if it is writeable). ---------------------------------------------------------------------------- 4.73. How do I specify hexadecimal and octal integers? To specify an octal digit, precede the octal value with a zero. For example, to set the variable "a" to the octal value "10" (8 in decimal), type: >>> a = 010 To verify that this works, you can type "a" and hit enter while in the interpreter, which will cause Python to spit out the current value of "a" in decimal: >>> a 8 Hexadecimal is just as easy. Simply precede the hexadecimal number with a zero, and then a lower or uppercase "x". Hexadecimal digits can be specified in lower or uppercase. For example, in the Python interpreter: >>> a = 0xa5 >>> a 165 >>> b = 0XB2 >>> b 178 ---------------------------------------------------------------------------- 4.74. How to get a single keypress at a time? For Windows, see question 8.2. Here is an answer for Unix. There are several solutions; some involve using curses, which is a pretty big thing to learn. Here's a solution without curses, due to Andrew Kuchling (adapted from code to do a PGP-style randomness pool): import termios, TERMIOS, sys, os fd = sys.stdin.fileno() old = termios.tcgetattr(fd) new = termios.tcgetattr(fd) new[3] = new[3] & ~TERMIOS.ICANON & ~TERMIOS.ECHO new[6][TERMIOS.VMIN] = 1 new[6][TERMIOS.VTIME] = 0 termios.tcsetattr(fd, TERMIOS.TCSANOW, new) s = '' # We'll save the characters typed and add them to the pool. try: while 1: c = os.read(fd, 1) print "Got character", `c` s = s+c finally: termios.tcsetattr(fd, TERMIOS.TCSAFLUSH, old) You need the termios module for any of this to work, and I've only tried it on Linux, though it should work elsewhere. It turns off stdin's echoing and disables canonical mode, and then reads a character at a time from stdin, noting the time after each keystroke. ---------------------------------------------------------------------------- 4.75. How can I overload constructors (or methods) in Python? (This actually applies to all methods, but somehow the question usually comes up first in the context of constructors.) Where in C++ you'd write class C { C() { cout << "No arguments\n"; } C(int i) { cout << "Argument is " << i << "\n"; } } in Python you have to write a single constructor that catches all cases using default arguments. For example: class C: def __init__(self, i=None): if i is None: print "No arguments" else: print "Argument is", i This is not entirely equivalent, but close enough in practice. You could also try a variable-length argument list, e.g. def __init__(self, *args): .... The same approach works for all method definitions. ---------------------------------------------------------------------------- 4.76. How do I pass keyword arguments from one method to another? Use apply. For example: class Account: def __init__(self, **kw): self.accountType = kw.get('accountType') self.balance = kw.get('balance') class CheckingAccount(Account): def __init__(self, **kw): kw['accountType'] = 'checking' apply(Account.__init__, (self,), kw) myAccount = CheckingAccount(balance=100.00) ---------------------------------------------------------------------------- 4.77. What module should I use to help with generating HTML? Check out HTMLgen written by Robin Friedrich. It's a class library of objects corresponding to all the HTML 3.2 markup tags. It's used when you are writing in Python and wish to synthesize HTML pages for generating a web or for CGI forms, etc. It can be found in the FTP contrib area on python.org or on the Starship. Use the search engines there to locate the latest version. It might also be useful to consider DocumentTemplate, which offers clear separation between Python code and HTML code. DocumentTemplate is part of the Bobo objects publishing system (http:/www.digicool.com/releases) but can be used independantly of course! ---------------------------------------------------------------------------- 4.78. How do I create documentation from doc strings? Use gendoc, by Daniel Larson. See http://starship.skyport.net/crew/danilo/ It can create HTML from the doc strings in your Python source code. ---------------------------------------------------------------------------- 4.79. How do I read (or write) binary data? For complex data formats, it's best to use use the struct module. It's documented in the library reference. It allows you to take a string read from a file containing binary data (usually numbers) and convert it to Python objects; and vice versa. For example, the following code reads two 2-byte integers and one 4-byte integer in big-endian format from a file: import struct f = open(filename, "rb") # Open in binary mode for portability s = f.read(8) x, y, z = struct.unpack(">hhl", s) The '>' in the format string forces bin-endian data; the letter 'h' reads one "short integer" (2 bytes), and 'l' reads one "long integer" (4 bytes) from the string. For data that is more regular (e.g. a homogeneous list of ints or floats), you can also use the array module, also documented in the library reference. ---------------------------------------------------------------------------- 4.80. I can't get key bindings to work in Tkinter An oft-heard complaint is that event handlers bound to events with the bind() method don't get handled even when the appropriate key is pressed. The most common cause is that the widget to which the binding applies doesn't have "keyboard focus". Check out the Tk documentation for the focus command. Usually a widget is given the keyboard focus by clicking in it (but not for labels; see the taketocus option). ---------------------------------------------------------------------------- 4.81. "import crypt" fails [Unix] Starting with Python 1.5, the crypt module is disabled by default. In order to enable it, you must go into the Python source tree and edit the file Modules/Setup to enable it (remove a '#' sign in front of the line starting with '#crypt'). Then rebuild. You may also have to add the string '-lcrypt' to that same line. ---------------------------------------------------------------------------- 4.82. Are there coding standards or a style guide for Python programs? Yes, Guido has written the "Python Style Guide". See http://www.python.org/doc/essays/styleguide.html ---------------------------------------------------------------------------- 4.83. How do I freeze Tkinter applications? Freeze is a tool to create stand-alone applications (see 4.28). When freezing Tkinter applications, the applications will not be truly stand-alone, as the application will still need the tcl and tk libraries. One solution is to ship the application with the tcl and tk libraries, and point to them at run-time using the TCL_LIBRARY and TK_LIBRARY environment variables. To get truly stand-alone applications, the Tcl scripts that form the library have to be integrated into the application as well. One tool supporting that is SAM (stand-alone modules), which is part of the Tix distribution (http://tix.mne.com). Build Tix with SAM enabled, perform the appropriate call to Tclsam_init etc inside Python's Modules/tkappinit.c, and link with libtclsam and libtksam (you might include the Tix libraries as well). ---------------------------------------------------------------------------- 4.84. How do I create static class data and static class methods? [Tim Peters, tim_one(a)email.msn.com] Static data (in the sense of C++ or Java) is easy; static methods (again in the sense of C++ or Java) are not supported directly. STATIC DATA For example, class C: count = 0 # number of times C.__init__ called def __init__(self): C.count = C.count + 1 def getcount(self): return C.count # or return self.count c.count also refers to C.count for any c such that isinstance(c, C) holds, unless overridden by c itself or by some class on the base-class search path from c.__class__ back to C. Caution: within a method of C, self.count = 42 creates a new and unrelated instance vrbl named "count" in self's own dict. So rebinding of a class-static data name needs the C.count = 314 form whether inside a method or not. STATIC METHODS Static methods (as opposed to static data) are unnatural in Python, because C.getcount returns an unbound method object, which can't be invoked without supplying an instance of C as the first argument. The intended way to get the effect of a static method is via a module-level function: def getcount(): return C.count If your code is structured so as to define one class (or tightly related class hierarchy) per module, this supplies the desired encapsulation. Several tortured schemes for faking static methods can be found by searching DejaNews. Most people feel such cures are worse than the disease. Perhaps the least obnoxious is due to Pekka Pessi (mailto:ppessi@hut.fi): # helper class to disguise function objects class _static: def __init__(self, f): self.__call__ = f class C: count = 0 def __init__(self): C.count = C.count + 1 def getcount(): return C.count getcount = _static(getcount) def sum(x, y): return x + y sum = _static(sum) C(); C() c = C() print C.getcount() # prints 3 print c.getcount() # prints 3 print C.sum(27, 15) # prints 42 ---------------------------------------------------------------------------- 4.85. __import__('x.y.z') returns <module 'x'>; how do I get z? Try __import__('x.y.z').y.z For more realistic situations, you may have to do something like m = __import__(s) for i in string.split(s, ".")[1:]: m = getattr(m, i) ---------------------------------------------------------------------------- 4.86. Basic thread wisdom If you write a simple test program like this: import thread def run(name, n): for i in range(n): print name, i for i in range(10): thread.start_new(run, (i, 100)) none of the threads seem to run! The reason is that as soon as the main thread exits, all threads are killed. A simple fix is to add a sleep to the end of the program, sufficiently long for all threads to finish: import thread, time def run(name, n): for i in range(n): print name, i for i in range(10): thread.start_new(run, (i, 100)) time.sleep(10) # <----------------------------! But now (on many platforms) the threads don't run in parallel, but appear to run sequentially, one at a time! The reason is that the OS thread scheduler doesn't start a new thread until the previous thread is blocked. A simple fix is to add a tiny sleep to the start of the run function: import thread, time def run(name, n): time.sleep(0.001) # <---------------------! for i in range(n): print name, i for i in range(10): thread.start_new(run, (i, 100)) time.sleep(10) Some more hints: Instead of using a time.sleep() call at the end, it's better to use some kind of semaphore mechanism. One idea is to use a the Queue module to create a queue object, let each thread append a token to the queue when it finishes, and let the main thread read as many tokens from the queue as there are threads. Use the threading module instead of the thread module. It's part of Python since version 1.5.1. It takes care of all these details, and has many other nice features too! ---------------------------------------------------------------------------- 4.87. Why doesn't closing sys.stdout (stdin, stderr) really close it? Python file objects are a high-level layer of abstraction on top of C streams, which in turn are a medium-level layer of abstraction on top of (among other things) low-level C file descriptors. For most file objects f you create in Python via the builtin "open" function, f.close() marks the Python file object as being closed from Python's point of view, and also arranges to close the underlying C stream. This happens automatically too, in f's destructor, when f becomes garbage. But stdin, stdout and stderr are treated specially by Python, because of the special status also given to them by C: doing sys.stdout.close() # ditto for stdin and stderr marks the Python-level file object as being closed, but does not close the associated C stream (provided sys.stdout is still bound to its default value, which is the stream C also calls "stdout"). To close the underlying C stream for one of these three, you should first be sure that's what you really want to do (e.g., you may confuse the heck out of extension modules trying to do I/O). If it is, use os.close: os.close(0) # close C's stdin stream os.close(1) # close C's stdout stream os.close(2) # close C's stderr stream ---------------------------------------------------------------------------- 4.88. What kinds of global value mutation are thread-safe? [adapted from c.l.py responses by Gordon McMillan & GvR] A global interpreter lock is used internally to ensure that only one thread runs in the Python VM at a time. In general, Python offers to switch among threads only between bytecode instructions (how frequently it offers to switch can be set via sys.setcheckinterval). Each bytecode instruction-- and all the C implementation code reached from it --is therefore atomic. In theory, this means an exact accounting requires an exact understanding of the PVM bytecode implementation. In practice, it means that operations on shared vrbls of builtin data types (ints, lists, dicts, etc) that "look atomic" really are. For example, these are atomic (L, L1, L2 are lists, D, D1, D2 are dicts, x, y are objects, i, j are ints): L.append(x) L1.extend(L2) x = L[i] x = L.pop() L1[i:j] = L2 L.sort() x = y x.field = y D[x] = y D1.update(D2) D.keys() These aren't: i = i+1 L.append(L[-1]) L[i] = L[j] D[x] = D[x] + 1 Note: operations that replace other objects may invoke those other objects' __del__ method when their reference count reaches zero, and that can affect things. This is especially true for the mass updates to dictionaries and lists. When in doubt, use a mutex! ---------------------------------------------------------------------------- 4.89. How do I modify a string in place? Strings are immutable (see question 6.2) so you cannot modify a string directly. If you need an object with this ability, try converting the string to a list or take a look at the array module. >>> s = "Hello, world" >>> a = list(s) >>> print a ['H', 'e', 'l', 'l', 'o', ',', ' ', 'w', 'o', 'r', 'l', 'd'] >>> a[7:] = list("there!") >>> import string >>> print string.join(a, '') 'Hello, there!' >>> import array >>> a = array.array('c', s) >>> print a array('c', 'Hello, world') >>> a[0] = 'y' ; print a array('c', 'yello world') >>> a.tostring() 'yello, world' ---------------------------------------------------------------------------- 4.90. How to pass on keyword/optional parameters/arguments Q: How can I pass on optional or keyword parameters from one function to another? A: Use 'apply', like: def f1(a, *b, **c): ... def f2(x, *y, **z): ... z['width']='14.3c' ... apply(f1, (a,)+b, c) ---------------------------------------------------------------------------- -- ----------- comp.lang.python.announce (moderated) ---------- Article Submission Address: python-announce(a)python.org Python Language Home Page: http://www.python.org/ Python Quick Help Index: http://www.python.org/Help.html ------------------------------------------------------------

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Python Language FAQ - Section 3
by Markus Fleck July 7, 1999

July 7, 1999

This FAQ newsgroup posting has been automatically converted from an HTML snapshot of the original Python FAQ; please refer to the original "Python FAQ Wizard" at <http://grail.cnri.reston.va.us/cgi-bin/faqw.py> if source code snippets given in this document do not work - incidentally some formatting information may have been lost during the conversion. ---------------------------------------------------------------------------- The whole Python FAQ - Section 3 Last changed on Mon Jun 28 19:36:09 1999 EDT (Entries marked with ** were changed within the last 24 hours; entries marked with * were changed within the last 7 days.) ---------------------------------------------------------------------------- 3. Building Python and Other Known Bugs 3.1. Is there a test set? 3.2. When running the test set, I get complaints about floating point operations, but when playing with floating point operations I cannot find anything wrong with them. 3.3. Link errors after rerunning the configure script. 3.4. The python interpreter complains about options passed to a script (after the script name). 3.5. When building on the SGI, make tries to run python to create glmodule.c, but python hasn't been built or installed yet. 3.6. I use VPATH but some targets are built in the source directory. 3.7. Trouble building or linking with the GNU readline library. 3.8. Trouble with socket I/O on older Linux 1.x versions. 3.9. Trouble with prototypes on Ultrix. 3.10. Other trouble building Python on platform X. 3.11. How to configure dynamic loading on Linux. 3.12. I can't get shared modules to work on Linux 2.0 (Slackware96)? 3.13. Trouble when making modules shared on Linux. 3.14. How to use threads on Linux. 3.15. Errors when linking with a shared library containing C++ code. 3.16. I built with tkintermodule.c enabled but get 'Tkinter not found' 3.17. I built with Tk 4.0 but Tkinter complains about the Tk version. 3.18. Compilation or link errors for the _tkinter module 3.19. I configured and built Python for Tcl/Tk but "import Tkinter" fails. 3.20. Tk doesn't work right on DEC Alpha. 3.21. Several common system calls are missing from the posix module. 3.22. ImportError: No module named string, on MS Windows. 3.23. Core dump on SGI when using the gl module. 3.24. "Initializer not a constant" while building DLL on MS-Windows 3.25. Output directed to a pipe or file disappears on Linux. 3.26. Syntax Errors all over the place in Linux with libc 5.4 3.27. Crash in XIO on Linux when using Tkinter. 3.28. How can I test if Tkinter is working? 3.29. Is there a way to get the interactive mode of the python interpreter to perform function/variable name completion? 3.30. Why is the Python interpreter not built as a shared library? 3.31. Build with GCC on Solaris 2.6 (SunOS 5.6) fails 3.32. Running "make clean" seems to leave problematic files that cause subsequent builds to fail. ---------------------------------------------------------------------------- 3. Building Python and Other Known Bugs ---------------------------------------------------------------------------- 3.1. Is there a test set? Sure. You can run it after building with "make test", or you can run it manuall with the command import test.autotest In 1.4 or earlier, use import autotest The test set doesn't test all features of Python, but it goes a long way to confirm that Python is actually working. NOTE: if "make test" fails, don't just mail the output to the newsgroup -- this doesn't give enough information to debug the problem. Instead, find out which test fails, and run that test manually from an interactive interpreter. For example, if "make test" reports that test_spam fails, try this interactively: import test.test_spam This generally produces more verbose output which can be diagnosed to debug the problem. ---------------------------------------------------------------------------- 3.2. When running the test set, I get complaints about floating point operations, but when playing with floating point operations I cannot find anything wrong with them. The test set makes occasional unwarranted assumptions about the semantics of C floating point operations. Until someone donates a better floating point test set, you will have to comment out the offending floating point tests and execute similar tests manually. ---------------------------------------------------------------------------- 3.3. Link errors after rerunning the configure script. It is generally necessary to run "make clean" after a configuration change. ---------------------------------------------------------------------------- 3.4. The python interpreter complains about options passed to a script (after the script name). You are probably linking with GNU getopt, e.g. through -liberty. Don't. The reason for the complaint is that GNU getopt, unlike System V getopt and other getopt implementations, doesn't consider a non-option to be the end of the option list. A quick (and compatible) fix for scripts is to add "--" to the interpreter, like this: #! /usr/local/bin/python -- You can also use this interactively: python -- script.py [options] Note that a working getopt implementation is provided in the Python distribution (in Python/getopt.c) but not automatically used. ---------------------------------------------------------------------------- 3.5. When building on the SGI, make tries to run python to create glmodule.c, but python hasn't been built or installed yet. Comment out the line mentioning glmodule.c in Setup and build a python without gl first; install it or make sure it is in your $PATH, then edit the Setup file again to turn on the gl module, and make again. You don't need to do "make clean"; you do need to run "make Makefile" in the Modules subdirectory (or just run "make" at the toplevel). ---------------------------------------------------------------------------- 3.6. I use VPATH but some targets are built in the source directory. On some systems (e.g. Sun), if the target already exists in the source directory, it is created there instead of in the build directory. This is usually because you have previously built without VPATH. Try running "make clobber" in the source directory. ---------------------------------------------------------------------------- 3.7. Trouble building or linking with the GNU readline library. Consider using readline 2.0. Some hints: You can use the GNU readline library to improve the interactive user interface: this gives you line editing and command history when calling python interactively. You need to configure and build the GNU readline library before running the configure script. Its sources are no longer distributed with Python; you can ftp them from any GNU mirror site, or from its home site ftp://slc2.ins.cwru.edu/pub/dist/readline-2.0.tar.gz (or a higher version number -- using version 1.x is not recommended). Pass the Python configure script the option --with-readline=DIRECTORY where DIRECTORY is the absolute pathname of the directory where you've built the readline library. Some hints on building and using the readline library: On SGI IRIX 5, you may have to add the following to rldefs.h: #ifndef sigmask #define sigmask(sig) (1L << ((sig)-1)) #endif On most systems, you will have to add #include "rldefs.h" to the top of several source files, and if you use the VPATH feature, you will have to add dependencies of the form foo.o: foo.c to the Makefile for several values of foo. The readline library requires use of the termcap library. A known problem with this is that it contains entry points which cause conflicts with the STDWIN and SGI GL libraries. The STDWIN conflict can be solved by adding a line saying '#define werase w_erase' to the stdwin.h file (in the STDWIN distribution, subdirectory H). The GL conflict has been solved in the Python configure script by a hack that forces use of the static version of the termcap library. Check the newsgroup gnu.bash.bug news:gnu.bash.bug for specific problems with the readline library (I don't read this group but I've been told that it is the place for readline bugs). ---------------------------------------------------------------------------- 3.8. Trouble with socket I/O on older Linux 1.x versions. Once you've built Python, use it to run the regen.py script in the Lib/linux1 directory. Apparently the files as distributed don't match the system headers on some Linux versions. ---------------------------------------------------------------------------- 3.9. Trouble with prototypes on Ultrix. Ultrix cc seems broken -- use gcc, or edit config.h to #undef HAVE_PROTOTYPES. ---------------------------------------------------------------------------- 3.10. Other trouble building Python on platform X. Please email the details to guido(a)cnri.reston.va.us and I'll look into it. Please provide as many details as possible. In particular, if you don't tell me what type of computer and what operating system (and version) you are using it will be difficult for me to figure out what is the matter. If you get a specific error message, please email it to me too. ---------------------------------------------------------------------------- 3.11. How to configure dynamic loading on Linux. This is now automatic as long as your Linux version uses the ELF object format (all recent Linuxes do). ---------------------------------------------------------------------------- 3.12. I can't get shared modules to work on Linux 2.0 (Slackware96)? This is a bug in the Slackware96 release. The fix is simple: Make sure that there is a link from /lib/libdl.so to /lib/libdl.so.1 so that the following links are setup: /lib/libdl.so -> /lib/libdl.so.1 /lib/libdl.so.1 -> /lib/libdl.so.1.7.14 You may have to rerun the configure script, after rm'ing the config.cache file, before you attempt to rebuild python after this fix. ---------------------------------------------------------------------------- 3.13. Trouble when making modules shared on Linux. This happens when you have built Python for static linking and then enable *shared* in the Setup file. Shared library code must be compiled with "-fpic". If a .o file for the module already exist that was compiled for static linking, you must remove it or do "make clean" in the Modules directory. ---------------------------------------------------------------------------- 3.14. How to use threads on Linux. [Greg Stein] You need to have a very recent libc, or even better, get the LinuxThreads-0.5 distribution. Note that if you install LinuxThreads normally, then you shouldn't need to specify the directory to the -with-thread configuration switch. The configure script ought to find it without a problem. To make sure everything builds properly, do a "make clean", remove config.cache, re-run configure with that switch, and then build. [Andy Dustman] On glibc systems (i.e. RedHat 5.0+), LinuxThreads is obsoleted by POSIX threads (-lpthread). If you upgraded from an earlier RedHat, remove LinuxThreads with "rpm -e linuxthreads linuxthreads-devel". Then run configure using --with-thread as above. ---------------------------------------------------------------------------- 3.15. Errors when linking with a shared library containing C++ code. Link the main Python binary with C++. Change the definition of LINKCC in Modules/Makefile to be your C++ compiler. You may have to edit config.c slightly to make it compilable with C++. ---------------------------------------------------------------------------- 3.16. I built with tkintermodule.c enabled but get 'Tkinter not found' Tkinter.py (note: upper case T) lives in a subdirectory of Lib, Lib/tkinter. If you are using the default module search path, you probably didn't enable the line in the Modules/Setup file defining TKPATH; if you use the environment variable PYTHONPATH, you'll have to add the proper tkinter subdirectory. For Windows, see question 7.11. ---------------------------------------------------------------------------- 3.17. I built with Tk 4.0 but Tkinter complains about the Tk version. Several things could cause this. You most likely have a Tk 3.6 installation that wasn't completely eradicated by the Tk 4.0 installation (which tends to add "4.0" to its installed files). You may have the Tk 3.6 support library installed in the place where the Tk 4.0 support files should be (default /usr/local/lib/tk/); you may have compiled Python with the old tk.h header file (yes, this actually compiles!); you may actually have linked with Tk 3.6 even though Tk 4.0 is also around. Similar for Tcl 7.4 vs. Tcl 7.3. ---------------------------------------------------------------------------- 3.18. Compilation or link errors for the _tkinter module Most likely, there's a version mismatch between the Tcl/Tk header files (tcl.h and tk.h) and the Tcl/Tk libraries you are using e.g. "-ltk8.0" and "-ltcl8.0" arguments for _tkinter in the Setup file). It is possible to install several versions of the Tcl/Tk libraries, but there can only be one version of the tcl.h and tk.h header files. If the library doesn't match the header, you'll get problems, either when linking the module, or when importing it. Fortunately, the version number is clearly stated in each file, so this is easy to find. Reinstalling and using the latest version usually fixes the problem. (Also note that when compiling unpatched Python 1.5.1 against Tcl/Tk 7.6/4.2 or older, you get an error on Tcl_Finalize. See the 1.5.1 patch page at http://www.python.org/1.5/patches-1.5.1/.) ---------------------------------------------------------------------------- 3.19. I configured and built Python for Tcl/Tk but "import Tkinter" fails. Most likely, you forgot to enable the line in Setup that says "TKPATH=:$(DESTLIB)/tkinter". ---------------------------------------------------------------------------- 3.20. Tk doesn't work right on DEC Alpha. You probably compiled either Tcl, Tk or Python with gcc. Don't. For this platform, which has 64-bit integers, gcc is known to generate broken code. The standard cc (which comes bundled with the OS!) works. If you still prefer gcc, at least try recompiling with cc before reporting problems to the newsgroup or the author; if this fixes the problem, report the bug to the gcc developers instead. (As far as we know, there are no problem with gcc on other platforms -- the instabilities seem to be restricted to the DEC Alpha.) See also question 3.6. There's also a 64-bit bugfix for Tcl/Tk; see http://grail.cnri.reston.va.us/grail/info/patches/tk64bit.txt ---------------------------------------------------------------------------- 3.21. Several common system calls are missing from the posix module. Most likely, all test compilations run by the configure script are failing for some reason or another. Have a look in config.log to see what could be the reason. A common reason is specifying a directory to the --with-readline option that doesn't contain the libreadline.a file. ---------------------------------------------------------------------------- 3.22. ImportError: No module named string, on MS Windows. Most likely, your PYTHONPATH environment variable should be set to something like: set PYTHONPATH=c:\python;c:\python\lib;c:\python\scripts (assuming Python was installed in c:\python) ---------------------------------------------------------------------------- 3.23. Core dump on SGI when using the gl module. There are conflicts between entry points in the termcap and curses libraries and an entry point in the GL library. There's a hack of a fix for the termcap library if it's needed for the GNU readline library, but it doesn't work when you're using curses. Concluding, you can't build a Python binary containing both the curses and gl modules. ---------------------------------------------------------------------------- 3.24. "Initializer not a constant" while building DLL on MS-Windows Static type object initializers in extension modules may cause compiles to fail with an error message like "initializer not a constant". Fredrik Lundh <Fredrik.Lundh(a)image.combitech.se> explains: This shows up when building DLL under MSVC. There's two ways to address this: either compile the module as C++, or change your code to something like: statichere PyTypeObject bstreamtype = { PyObject_HEAD_INIT(NULL) /* must be set by init function */ 0, "bstream", sizeof(bstreamobject), ... void initbstream() { /* Patch object type */ bstreamtype.ob_type = &PyType_Type; Py_InitModule("bstream", functions); ... } ---------------------------------------------------------------------------- 3.25. Output directed to a pipe or file disappears on Linux. Some people have reported that when they run their script interactively, it runs great, but that when they redirect it to a pipe or file, no output appears. % python script.py ...some output... % python script.py >file % cat file % # no output % python script.py | cat % # no output % Nobody knows what causes this, but it is apparently a Linux bug. Most Linux users are not affected by this. There's at least one report of someone who reinstalled Linux (presumably a newer version) and Python and got rid of the problem; so this may be the solution. ---------------------------------------------------------------------------- 3.26. Syntax Errors all over the place in Linux with libc 5.4 ``I have installed python1.4 on my Linux system. When I try run the import statement I get the following error message:'' File "<stdin>", line 1 import sys ^ Syntax Error: "invalid syntax" Did you compile it yourself? This usually is caused by an incompatibility between libc 5.4.x and earlier libc's. In particular, programs compiled with libc 5.4 give incorrect results on systems which had libc 5.2 installed because the ctype.h file is broken. In this case, Python can't recognize which characters are letters and so on. The fix is to install the C library which was used when building the binary that you installed, or to compile Python yourself. When you do this, make sure the C library header files which get used by the compiler match the installed C library. [adapted from an answer by Martin v. Loewis] PS [adapted from Andreas Jung]: If you have upgraded to libc 5.4.x, and the problem persists, check your library path for an older version of libc. Try to clean update libc with the libs and the header files and then try to recompile all. ---------------------------------------------------------------------------- 3.27. Crash in XIO on Linux when using Tkinter. When Python is built with threads under Linux, use of Tkinter can cause crashes like the following: >>> from Tkinter import * >>> root = Tk() XIO: fatal IO error 0 (Unknown error) on X server ":0.0" after 45 requests (40 known processed) with 1 events remaining. The reason is that the default Xlib is not built with support for threads. If you rebuild Xlib with threads enabled the problems go away. Alternatively, you can rebuild Python without threads ("make clean" first!). (Disclaimer: this is from memory.) ---------------------------------------------------------------------------- 3.28. How can I test if Tkinter is working? Try the following: python >>> import _tkinter >>> import Tkinter >>> Tkinter._test() This should pop up a window with two buttons, one "Click me" and one "Quit". If the first statement (import _tkinter) fails, your Python installation probably has not been configured to support Tcl/Tk. On Unix, if you have installed Tcl/Tk, you have to rebuild Python after editing the Modules/Setup file to enable the _tkinter module and the TKPATH environment variable. It is also possible to get complaints about Tcl/Tk version number mismatches or missing TCL_LIBRARY or TK_LIBRARY environment variables. These have to do with Tcl/Tk installation problems. A common problem is to have installed versions of tcl.h and tk.h that don't match the installed version of the Tcl/Tk libraries; this usually results in linker errors or (when using dynamic loading) complaints about missing symbols during loading the shared library. ---------------------------------------------------------------------------- 3.29. Is there a way to get the interactive mode of the python interpreter to perform function/variable name completion? (From a posting by Guido van Rossum) On Unix, if you have enabled the readline module (i.e. if Emacs-style command line editing and bash-style history works for you), you can add this by importing the undocumented standard library module "rlcompleter". When completing a simple identifier, it completes keywords, built-ins and globals in __main__; when completing NAME.NAME..., it evaluates (!) the expression up to the last dot and completes its attributes. This way, you can do "import string", type "string.", hit the completion key twice, and see the list of names defined by the string module. Tip: to use the tab key as the completion key, call readline.parse_and_bind("tab: complete") You can put this in a ~/.pythonrc file, and set the PYTHONSTARTUP environment variable to ~/.pythonrc. This will cause the completion to be enabled whenever you run Python interactively. Notes (see the docstring for rlcompleter.py for more information): * The evaluation of the NAME.NAME... form may cause arbitrary application defined code to be executed if an object with a __getattr__ hook is found. Since it is the responsibility of the application (or the user) to enable this feature, I consider this an acceptable risk. More complicated expressions (e.g. function calls or indexing operations) are not evaluated. * GNU readline is also used by the built-in functions input() and raw_input(), and thus these also benefit/suffer from the complete features. Clearly an interactive application can benefit by specifying its own completer function and using raw_input() for all its input. * When stdin is not a tty device, GNU readline is never used, and this module (and the readline module) are silently inactive. ---------------------------------------------------------------------------- 3.30. Why is the Python interpreter not built as a shared library? (This is a Unix question; on Mac and Windows, it is a shared library.) It's just a nightmare to get this to work on all different platforms. Shared library portability is a pain. And yes, I know about GNU libtool -- but it requires me to use its conventions for filenames etc, and it would require a complete and utter rewrite of all the makefile and config tools I'm currently using. In practice, few applications embed Python -- it's much more common to have Python extensions, which already are shared libraries. Also, serious embedders often want total control over which Python version and configuration they use so they wouldn't want to use a standard shared library anyway. So while the motivation of saving space when lots of apps embed Python is nice in theory, I doubt that it will save much in practice. (Hence the low priority I give to making a shared library.) ---------------------------------------------------------------------------- 3.31. Build with GCC on Solaris 2.6 (SunOS 5.6) fails If you have upgraded Solaris 2.5 or 2.5.1 to Solaris 2.6, but you have not upgraded your GCC installation, the compile may fail, e.g. like this: In file included from /usr/include/sys/stream.h:26, from /usr/include/netinet/in.h:38, from /usr/include/netdb.h:96, from ./socketmodule.c:121: /usr/include/sys/model.h:32: #error "No DATAMODEL_NATIVE specified" Solution: rebuild GCC for Solaris 2.6. You might be able to simply re-run fixincludes, but people have had mixed success with doing that. ---------------------------------------------------------------------------- 3.32. Running "make clean" seems to leave problematic files that cause subsequent builds to fail. Use "make clobber" instead. Use "make clean" to reduce the size of the source/build directory after you're happy with your build and installation. If you have already tried to build python and you'd like to start over, you should use "make clobber". It does a "make clean" and also removes files such as the partially built Python library from a previous build. ---------------------------------------------------------------------------- -- ----------- comp.lang.python.announce (moderated) ---------- Article Submission Address: python-announce(a)python.org Python Language Home Page: http://www.python.org/ Python Quick Help Index: http://www.python.org/Help.html ------------------------------------------------------------

1 0

Python Language FAQ - Section 2
by Markus Fleck July 7, 1999

July 7, 1999

This FAQ newsgroup posting has been automatically converted from an HTML snapshot of the original Python FAQ; please refer to the original "Python FAQ Wizard" at <http://grail.cnri.reston.va.us/cgi-bin/faqw.py> if source code snippets given in this document do not work - incidentally some formatting information may have been lost during the conversion. ---------------------------------------------------------------------------- The whole Python FAQ - Section 2 Last changed on Mon Jun 28 19:36:09 1999 EDT (Entries marked with ** were changed within the last 24 hours; entries marked with * were changed within the last 7 days.) ---------------------------------------------------------------------------- 2. Python in the real world 2.1. How many people are using Python? 2.2. Have any significant projects been done in Python? 2.3. Are there any commercial projects going on using Python? 2.4. How stable is Python? 2.5. What new developments are expected for Python in the future? 2.6. Is it reasonable to propose incompatible changes to Python? 2.7. What is the future of Python? 2.8. What is the PSA, anyway? 2.9. How do I join the PSA? 2.10. What are the benefits of joining the PSA? 2.11. Is Python Y2K (Year 2000) Compliant? ---------------------------------------------------------------------------- 2. Python in the real world ---------------------------------------------------------------------------- 2.1. How many people are using Python? I don't know, but the maximum number of simultaneous subscriptions to the Python mailing list before it was gatewayed into the newsgroup was about 180 (several of which were local redistribution lists). I believe that many active Python users don't bother to subscribe to the list, and now that there's a newsgroup the mailing list subscription is even less meaningful. I see new names on the newsgroup all the time and my best guess is that there are currently at least several thousands of users. Another statistic is the number of accesses to the Python WWW server. Have a look at http://www.python.org/stats/. ---------------------------------------------------------------------------- 2.2. Have any significant projects been done in Python? At CWI (the former home of Python), we have written a 20,000 line authoring environment for transportable hypermedia presentations, a 5,000 line multimedia teleconferencing tool, as well as many many smaller programs. At CNRI (Python's new home), we have written two large applications: Grail, a fully featured web browser (see http://grail.cnri.reston.va.us), and the Knowbot Operating Environment, a distributed environment for mobile code. The University of Virginia uses Python to control a virtual reality engine. See http://alice.cs.cmu.edu. The ILU project at Xerox PARC can generate Python glue for ILU interfaces. See ftp://ftp.parc.xerox.com/pub/ilu/ilu.html. ILU is a free CORBA compliant ORB which supplies distributed object connectivity to a host of platforms using a host of languages. Mark Hammond and Greg Stein and others are interfacing Python to Microsoft's COM and ActiveX architectures. This means, among other things, that Python may be used in active server pages or as a COM controller (for example to automatically extract from or insert information into Excel or MSAccess or any other COM aware application). Mark claims Python can even be a ActiveX scripting host (which means you could embed JScript inside a Python application, if you had a strange sense of humor). Python/AX/COM is distributed as part of the PythonWin distribution. The University of California, Irvine uses a student administration system called TELE-Vision written entirely in Python. Contact: Ray Price rlprice(a)uci.edu. The Melbourne Cricket Ground (MCG) in Australia (a 100,000+ person venue) has it's scoreboard system written largely in Python on MS Windows. Python expressions are used to create almost every scoring entry that appears on the board. The move to Python/C++ away from exclusive C++ has provided a level of functionality that would simply not have been viable otherwise. See also the next question. If you have done a significant project in Python that you'd like to be included in the list above, send me email! ---------------------------------------------------------------------------- 2.3. Are there any commercial projects going on using Python? Yes, there's lots of commercial activity using Python. See http://www.python.org/psa/Users.html for a list. ---------------------------------------------------------------------------- 2.4. How stable is Python? Very stable. While the current version number would suggest it is in the early stages of development, in fact new, stable releases (numbered 0.9.x through 1.4) have been coming out roughly every 3 to 6 or 12 months for the past four years. ---------------------------------------------------------------------------- 2.5. What new developments are expected for Python in the future? Follow the newsgroup discussions! The workshop proceedings (http://www.python.org/workshops/) may also contain interesting looks into the future. ---------------------------------------------------------------------------- 2.6. Is it reasonable to propose incompatible changes to Python? In general, no. There are already millions of lines of Python code around the world, so any changes in the language that invalidates more than a very small fraction of existing programs has to be frowned upon. Even if you can provide a conversion program, there still is the problem of updating all documentation. Providing a gradual upgrade path is the only way if a feature has to be changed. ---------------------------------------------------------------------------- 2.7. What is the future of Python? If I knew, I'd be rich :-) Seriously, the formation of the PSA (Python Software Activity, see http://www.python.org/psa/) ensures some kind of support even in the (unlikely!) event that I'd be hit by a bus (actually, here in the U.S., a car accident would be more likely :-), were to join a nunnery, or would be head-hunted. A large number of Python users have become experts at Python programming as well as maintenance of the implementation, and would easily fill the vacuum created by my disappearance. In the meantime, I have no plans to disappear -- rather, I am committed to improving Python, and my current benefactor, CNRI (see http://www.cnri.reston.va.us) is just as committed to continue its support of Python and the PSA. In fact, we have great plans for Python -- we just can't tell yet! ---------------------------------------------------------------------------- 2.8. What is the PSA, anyway? The Python Software Activity http://www.python.org/psa/ was created by a number of Python aficionados who want Python to be more than the product and responsibility of a single individual. It has found a home at CNRI http://www.cnri.reston.va.us. Anybody who wishes Python well should join the PSA. ---------------------------------------------------------------------------- 2.9. How do I join the PSA? The full scoop is available on the web, see http://www.python.org/psa/regtempl.html. Summary: send a check of at least $50 to CNRI/PSA, 1895 Preston White Drive, Suite 100, in Reston, VA 20191. Full-time students pay $25. Prices drop by half the second half of the fiscal year (April - September). Companies can join for a mere $500. Pets may join for only $15! ---------------------------------------------------------------------------- 2.10. What are the benefits of joining the PSA? Like National Public Radio, without your support, Python will wither. If you join, your name will be mentioned on the PSA's web server. Workshops organized by the PSA http://www.python.org/workshops/ are only accessible to PSA members (you can join at the door). The PSA is working on additional benefits, such as reduced prices for books and software, and early access to alpha versions of Python. (The latter has been realized -- the 1.5 alpha testing program is accessible only to PSA members.) You might also consider to become a member of the starship project. It is a free group of Python enthusiasts, and you get a free account. They just happen to admit only PSA members. Check out http://starship.python.net for further information. ---------------------------------------------------------------------------- 2.11. Is Python Y2K (Year 2000) Compliant? Since Python is available free of charge, I don't want to make any absolute guarantees. If there is a problem that I didn't foresee, I don't want to be sued for damages. That said, I'm pretty convinced that there are no Y2K problems anywhere in the core distribution, either 1.5 or 1.4. Python does few date manipulations, and what it does is all based on the Unix representation for time (even on non-Unix systems) which uses seconds since 1970 and won't overflow until 2038. ---------------------------------------------------------------------------- -- ----------- comp.lang.python.announce (moderated) ---------- Article Submission Address: python-announce(a)python.org Python Language Home Page: http://www.python.org/ Python Quick Help Index: http://www.python.org/Help.html ------------------------------------------------------------

1 0

Python Language FAQ - Section 1
by Markus Fleck July 7, 1999

July 7, 1999

This FAQ newsgroup posting has been automatically converted from an HTML snapshot of the original Python FAQ; please refer to the original "Python FAQ Wizard" at <http://grail.cnri.reston.va.us/cgi-bin/faqw.py> if source code snippets given in this document do not work - incidentally some formatting information may have been lost during the conversion. ---------------------------------------------------------------------------- The whole Python FAQ - Section 1 Last changed on Mon Jun 28 19:36:09 1999 EDT (Entries marked with ** were changed within the last 24 hours; entries marked with * were changed within the last 7 days.) ---------------------------------------------------------------------------- 1. General information and availability 1.1. What is Python? 1.2. Why is it called Python? 1.3. How do I obtain a copy of the Python source? 1.4. How do I get documentation on Python? 1.5. Are there other ftp sites that mirror the Python distribution? 1.6. Is there a newsgroup or mailing list devoted to Python? 1.7. Is there a WWW page devoted to Python? 1.8. Is the Python documentation available on the WWW? 1.9. Are there any books on Python? 1.10. Are there any published articles about Python that I can reference? 1.11. Are there short introductory papers or talks on Python? 1.12. How does the Python version numbering scheme work? 1.13. How do I get a beta test version of Python? 1.14. Are there copyright restrictions on the use of Python? 1.15. Why was Python created in the first place? 1.16. Do I have to like "Monty Python's Flying Circus"? 1.17. What is Python good for? 1.18. Can I use the FAQ Wizard software to maintain my own FAQ? 1.19. Which editor has good support for editing Python source code? ---------------------------------------------------------------------------- 1. General information and availability ---------------------------------------------------------------------------- 1.1. What is Python? Python is an interpreted, interactive, object-oriented programming language. It incorporates modules, exceptions, dynamic typing, very high level dynamic data types, and classes. Python combines remarkable power with very clear syntax. It has interfaces to many system calls and libraries, as well as to various window systems, and is extensible in C or C++. It is also usable as an extension language for applications that need a programmable interface. Finally, Python is portable: it runs on many brands of UNIX, on the Mac, and on PCs under MS-DOS, Windows, Windows NT, and OS/2. To find out more, the best thing to do is to start reading the tutorial from the documentation set (see a few questions further down). See also question 1.17 (what is Python good for). ---------------------------------------------------------------------------- 1.2. Why is it called Python? Apart from being a computer scientist, I'm also a fan of "Monty Python's Flying Circus" (a BBC comedy series from the seventies, in the -- unlikely -- case you didn't know). It occurred to me one day that I needed a name that was short, unique, and slightly mysterious. And I happened to be reading some scripts from the series at the time... So then I decided to call my language Python. But Python is not a joke. And don't you associate it with dangerous reptiles either! (If you need an icon, use an image of the 16-ton weight from the TV series or of a can of SPAM :-) ---------------------------------------------------------------------------- 1.3. How do I obtain a copy of the Python source? The latest complete Python source distribution is always available by anonymous ftp, e.g. ftp://ftp.python.org/pub/python/src/python1.4.tar.gz. It is a gzipped tar file containing the complete C source, LaTeX documentation, Python library modules, example programs, and several useful pieces of freely distributable software. This will compile and run out of the box on most UNIX platforms. (See section 7 for non-UNIX information.) An index of said ftp directory can be found in the file INDEX. An HTML version of the index can be found in the file index.html, ftp://ftp.python.org/pub/python/index.html. ---------------------------------------------------------------------------- 1.4. How do I get documentation on Python? All documentation is available on-line, starting at http://www.python.org/doc/. The LaTeX source for the documentation is part of the source distribution. If you don't have LaTeX, the latest Python documentation set is available, in various formats like postscript and html, by anonymous ftp - visit the above URL for links to the current versions. PostScript for a high-level description of Python is in the file nluug-paper.ps (a separate file on the ftp site). ---------------------------------------------------------------------------- 1.5. Are there other ftp sites that mirror the Python distribution? The following anonymous ftp sites keep mirrors of the Python distribution: USA: ftp://ftp.python.org/pub/python/ ftp://gatekeeper.dec.com/pub/plan/python/ ftp://ftp.uu.net/languages/python/ ftp://ftp.wustl.edu/graphics/graphics/sgi-stuff/python/ ftp://ftp.sterling.com/programming/languages/python/ ftp://uiarchive.cso.uiuc.edu/pub/lang/python/ ftp://ftp.pht.com/mirrors/python/python/ ftp://ftp.cdrom.com/pub/python/ Europe: ftp://ftp.cwi.nl/pub/python/ ftp://ftp.funet.fi/pub/languages/python/ ftp://ftp.sunet.se/pub/lang/python/ ftp://unix.hensa.ac.uk/mirrors/uunet/languages/python/ ftp://ftp.lip6.fr/pub/python/ ftp://sunsite.cnlab-switch.ch/mirror/python/ ftp://ftp.informatik.tu-muenchen.de/pub/comp/programming/languages/python/ Australia: ftp://ftp.dstc.edu.au/pub/python/ ---------------------------------------------------------------------------- 1.6. Is there a newsgroup or mailing list devoted to Python? There is a newsgroup, comp.lang.python, and a mailing list. The newsgroup and mailing list are gatewayed into each other -- if you can read news it's unnecessary to subscribe to the mailing list. To subscribe to the mailing list (python-list(a)python.org) visit its Mailman webpage at http://www.python.org/mailman/listinfo/python-list More info about the newsgroup and mailing list, and about other lists, can be found at http://www.python.org/psa/MailingLists.html. Archives of the newsgroup are kept by Deja News and accessible through the "Python newsgroup search" web page, http://www.python.org/search/search_news.html. This page also contains pointer to other archival collections. ---------------------------------------------------------------------------- 1.7. Is there a WWW page devoted to Python? Yes, http://www.python.org/ is the official Python home page. ---------------------------------------------------------------------------- 1.8. Is the Python documentation available on the WWW? Yes, see http://www.python.org/ (Python's home page). It contains pointers to hypertext versions of the whole documentation set (as hypertext, not just PostScript). If you wish to browse this collection of HTML files on your own machine, it is available bundled up by anonymous ftp, e.g. ftp://ftp.python.org/pub/python/doc/html.tar.gz. An Emacs-INFO set containing the library manual is also available by ftp, e.g. ftp://ftp.python.org/pub/python/doc/lib-info.tar.gz. ---------------------------------------------------------------------------- 1.9. Are there any books on Python? Yes, several: + Internet Programming with Python by Aaron Watters, Guido van Rossum, and James Ahlstrom MIS Press/Henry Holt publishers ISBN: 1-55851-484-8 First published October, 1996 + Programming Python by Mark Lutz O'Reilly & Associates ISBN: 1-56592-197-6 First published October, 1996 + Das Python-Buch (in German) by Martin von Loewis and Nils Fischbeck Addison-Wesley-Longman, 1997 ISBN: 3-8273-1110-1 More are coming: + Python Pocket Reference by Mark Lutz O'Reilly & Associates ISBN 1-56592-500-9 1st Edition October 1998 (est.) 80 pages (est.), $6.95 (est.) ---------------------------------------------------------------------------- 1.10. Are there any published articles about Python that I can reference? If you can't reference the web site, and you don't want to reference the books (see previous question), there are several articles on Python that you could reference. Most publications about Python are collected on the Python web site: http://www.python.org/doc/Publications.html It is no longer recommended to reference this very old article by Python's author: Guido van Rossum and Jelke de Boer, "Interactively Testing Remote Servers Using the Python Programming Language", CWI Quarterly, Volume 4, Issue 4 (December 1991), Amsterdam, pp 283-303. ---------------------------------------------------------------------------- 1.11. Are there short introductory papers or talks on Python? There are several - you can find links to some of them collected at http://www.python.org/doc/Hints.html#intros. ---------------------------------------------------------------------------- 1.12. How does the Python version numbering scheme work? Python versions are numbered A.B.C or A.B. A is the major version number -- it is only incremented for major changes in functionality or source structure. B is the minor version number, incremented for less earth-shattering changes to a release. C is the patchlevel -- it is incremented for each new patch release. Not all releases have patch releases. Note that in the past, patches have added significant changes; in fact the changeover from 0.9.9 to 1.0.0 was the first time that either A or B changed! Beta versions have an additional suffix of "betaN" for some small number N. Note that (for instance) all versions labeled 1.4betaN precede the actual release of 1.4. 1.4b3 is short for 1.4beta3. ---------------------------------------------------------------------------- 1.13. How do I get a beta test version of Python? If there are any beta releases, they are published in the normal source directory (e.g. ftp://ftp.python.org/pub/python/src/). Alpha releases are only open to PSA members. See http://www.python.org/psa/ for information on how to join ($50/year). ---------------------------------------------------------------------------- 1.14. Are there copyright restrictions on the use of Python? Hardly. You can do anything you want with the source, as long as you leave the copyrights in, and display those copyrights in any documentation about Python that you produce. Also, don't use the author's institute's name in publicity without prior written permission, and don't hold them responsible for anything (read the actual copyright for a precise legal wording). In particular, if you honor the copyright rules, it's OK to use Python for commercial use, to sell copies of Python in source or binary form, or to sell products that enhance Python or incorporate Python (or part of it) in some form. I would still like to know about all commercial use of Python! ---------------------------------------------------------------------------- 1.15. Why was Python created in the first place? Here's a very brief summary of what got me started: I had extensive experience with implementing an interpreted language in the ABC group at CWI, and from working with this group I had learned a lot about language design. This is the origin of many Python features, including the use of indentation for statement grouping and the inclusion of very-high-level data types (although the details are all different in Python). I had a number of gripes about the ABC language, but also liked many of its features. It was impossible to extend the ABC language (or its implementation) to remedy my complaints -- in fact its lack of extensibility was one of its biggest problems. I had some experience with using Modula-2+ and talked with the designers of Modula-3 (and read the M3 report). M3 is the origin of the syntax and semantics used for exceptions, and some other Python features. I was working in the Amoeba distributed operating system group at CWI. We needed a better way to do system administration than by writing either C programs or Bourne shell scripts, since Amoeba had its own system call interface which wasn't easily accessible from the Bourne shell. My experience with error handling in Amoeba made me acutely aware of the importance of exceptions as a programming language feature. It occurred to me that a scripting language with a syntax like ABC but with access to the Amoeba system calls would fill the need. I realized that it would be foolish to write an Amoeba-specific language, so I decided that I needed a language that was generally extensible. During the 1989 Christmas holidays, I had a lot of time on my hand, so I decided to give it a try. During the next year, while still mostly working on it in my own time, Python was used in the Amoeba project with increasing success, and the feedback from colleagues made me add many early improvements. In February 1991, after just over a year of development, I decided to post to USENET. The rest is in the Misc/HISTORY file. ---------------------------------------------------------------------------- 1.16. Do I have to like "Monty Python's Flying Circus"? No, but it helps. Pythonistas like the occasional reference to SPAM, and of course, nobody expects the Spanish Inquisition The two main reasons to use Python are: - Portable - Easy to learn The three main reasons to use Python are: - Portable - Easy to learn - Powerful standard library (And nice red uniforms.) And remember, there is no rule six. ---------------------------------------------------------------------------- 1.17. What is Python good for? Python is used in many situations where a great deal of dynamism, ease of use, power, and flexibility are required. In the area of basic text manipulation core Python (without any non-core extensions) is easier to use and is roughly as fast as just about any language, and this makes Python good for many system administration type tasks and for CGI programming and other application areas that manipulate text and strings and such. When augmented with standard extensions (such as PIL, COM, Numeric, oracledb, kjbuckets, tkinter, win32api, etc.) or special purpose extensions (that you write, perhaps using helper tools such as SWIG, or using object protocols such as ILU/CORBA or COM) Python becomes a very convenient "glue" or "steering" language that helps make heterogeneous collections of unrelated software packages work together. For example by combining Numeric with oracledb you can help your SQL database do statistical analysis, or even Fourier transforms. One of the features that makes Python excel in the "glue language" role is Python's simple, usable, and powerful C language runtime API. Many developers also use Python extensively as a graphical user interface development aide. ---------------------------------------------------------------------------- 1.18. Can I use the FAQ Wizard software to maintain my own FAQ? Sure. Version 1.0 is distributed in the Tools subdirectory of the Python 1.5 source release at http://www.python.org/ftp/python/src/py152.tgz ---------------------------------------------------------------------------- 1.19. Which editor has good support for editing Python source code? On Unix, the first choice is Emacs/XEmacs. There's an elaborate mode for editing Python code, which is available from the Python source distribution (Misc/python-mode.el). It's also bundled with XEmacs (we're still working on legal details to make it possible to bundle it with FSF Emacs). And it has its own web page: http://www.python.org/emacs/python-mode/index.html There are many other choices, for Unix, Windows or Macintosh. Richard Jones compiled a table from postings on the Python newsgroup: http://www.bofh.asn.au/~richard/editors.html See also FAQ question 7.10 for some more Mac and Win options. ---------------------------------------------------------------------------- -- ----------- comp.lang.python.announce (moderated) ---------- Article Submission Address: python-announce(a)python.org Python Language Home Page: http://www.python.org/ Python Quick Help Index: http://www.python.org/Help.html ------------------------------------------------------------

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Python Language FAQ - Table of Contents
by Markus Fleck July 7, 1999

July 7, 1999

This FAQ newsgroup posting has been automatically converted from an HTML snapshot of the original Python FAQ; please refer to the original "Python FAQ Wizard" at <http://grail.cnri.reston.va.us/cgi-bin/faqw.py> if source code snippets given in this document do not work - incidentally some formatting information may have been lost during the conversion. ---------------------------------------------------------------------------- The whole Python FAQ - Table of Contents Last changed on Mon Jun 28 19:36:09 1999 EDT (Entries marked with ** were changed within the last 24 hours; entries marked with * were changed within the last 7 days.) ---------------------------------------------------------------------------- 1. General information and availability 1.1. What is Python? 1.2. Why is it called Python? 1.3. How do I obtain a copy of the Python source? 1.4. How do I get documentation on Python? 1.5. Are there other ftp sites that mirror the Python distribution? 1.6. Is there a newsgroup or mailing list devoted to Python? 1.7. Is there a WWW page devoted to Python? 1.8. Is the Python documentation available on the WWW? 1.9. Are there any books on Python? 1.10. Are there any published articles about Python that I can reference? 1.11. Are there short introductory papers or talks on Python? 1.12. How does the Python version numbering scheme work? 1.13. How do I get a beta test version of Python? 1.14. Are there copyright restrictions on the use of Python? 1.15. Why was Python created in the first place? 1.16. Do I have to like "Monty Python's Flying Circus"? 1.17. What is Python good for? 1.18. Can I use the FAQ Wizard software to maintain my own FAQ? 1.19. Which editor has good support for editing Python source code? ---------------------------------------------------------------------------- 2. Python in the real world 2.1. How many people are using Python? 2.2. Have any significant projects been done in Python? 2.3. Are there any commercial projects going on using Python? 2.4. How stable is Python? 2.5. What new developments are expected for Python in the future? 2.6. Is it reasonable to propose incompatible changes to Python? 2.7. What is the future of Python? 2.8. What is the PSA, anyway? 2.9. How do I join the PSA? 2.10. What are the benefits of joining the PSA? 2.11. Is Python Y2K (Year 2000) Compliant? ---------------------------------------------------------------------------- 3. Building Python and Other Known Bugs 3.1. Is there a test set? 3.2. When running the test set, I get complaints about floating point operations, but when playing with floating point operations I cannot find anything wrong with them. 3.3. Link errors after rerunning the configure script. 3.4. The python interpreter complains about options passed to a script (after the script name). 3.5. When building on the SGI, make tries to run python to create glmodule.c, but python hasn't been built or installed yet. 3.6. I use VPATH but some targets are built in the source directory. 3.7. Trouble building or linking with the GNU readline library. 3.8. Trouble with socket I/O on older Linux 1.x versions. 3.9. Trouble with prototypes on Ultrix. 3.10. Other trouble building Python on platform X. 3.11. How to configure dynamic loading on Linux. 3.12. I can't get shared modules to work on Linux 2.0 (Slackware96)? 3.13. Trouble when making modules shared on Linux. 3.14. How to use threads on Linux. 3.15. Errors when linking with a shared library containing C++ code. 3.16. I built with tkintermodule.c enabled but get 'Tkinter not found' 3.17. I built with Tk 4.0 but Tkinter complains about the Tk version. 3.18. Compilation or link errors for the _tkinter module 3.19. I configured and built Python for Tcl/Tk but "import Tkinter" fails. 3.20. Tk doesn't work right on DEC Alpha. 3.21. Several common system calls are missing from the posix module. 3.22. ImportError: No module named string, on MS Windows. 3.23. Core dump on SGI when using the gl module. 3.24. "Initializer not a constant" while building DLL on MS-Windows 3.25. Output directed to a pipe or file disappears on Linux. 3.26. Syntax Errors all over the place in Linux with libc 5.4 3.27. Crash in XIO on Linux when using Tkinter. 3.28. How can I test if Tkinter is working? 3.29. Is there a way to get the interactive mode of the python interpreter to perform function/variable name completion? 3.30. Why is the Python interpreter not built as a shared library? 3.31. Build with GCC on Solaris 2.6 (SunOS 5.6) fails 3.32. Running "make clean" seems to leave problematic files that cause subsequent builds to fail. ---------------------------------------------------------------------------- 4. Programming in Python 4.1. Is there a source code level debugger with breakpoints, step, etc.? 4.2. Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)? (Also phrased as: Can I use a built-in type as base class?) 4.3. Is there a curses/termcap package for Python? 4.4. Is there an equivalent to C's onexit() in Python? 4.5. When I define a function nested inside another function, the nested function seemingly can't access the local variables of the outer function. What is going on? How do I pass local data to a nested function? 4.6. How do I iterate over a sequence in reverse order? 4.7. My program is too slow. How do I speed it up? 4.8. When I have imported a module, then edit it, and import it again (into the same Python process), the changes don't seem to take place. What is going on? 4.9. How do I find the current module name? 4.10. I have a module in which I want to execute some extra code when it is run as a script. How do I find out whether I am running as a script? 4.11. I try to run a program from the Demo directory but it fails with ImportError: No module named ...; what gives? 4.12. I have successfully built Python with STDWIN but it can't find some modules (e.g. stdwinevents). 4.13. What GUI toolkits exist for Python? 4.14. Are there any interfaces to database packages in Python? 4.15. Is it possible to write obfuscated one-liners in Python? 4.16. Is there an equivalent of C's "?:" ternary operator? 4.17. My class defines __del__ but it is not called when I delete the object. 4.18. How do I change the shell environment for programs called using os.popen() or os.system()? Changing os.environ doesn't work. 4.19. What is a class? 4.20. What is a method? 4.21. What is self? 4.22. What is an unbound method? 4.23. How do I call a method defined in a base class from a derived class that overrides it? 4.24. How do I call a method from a base class without using the name of the base class? 4.25. How can I organize my code to make it easier to change the base class? 4.26. How can I find the methods or attributes of an object? 4.27. I can't seem to use os.read() on a pipe created with os.popen(). 4.28. How can I create a stand-alone binary from a Python script? 4.29. What WWW tools are there for Python? 4.30. How do I run a subprocess with pipes connected to both input and output? 4.31. How do I call a function if I have the arguments in a tuple? 4.32. How do I enable font-lock-mode for Python in Emacs? 4.33. Is there a scanf() or sscanf() equivalent? 4.34. Can I have Tk events handled while waiting for I/O? 4.35. How do I write a function with output parameters (call by reference)? 4.36. Please explain the rules for local and global variables in Python. 4.37. How can I have modules that mutually import each other? 4.38. How do I copy an object in Python? 4.39. How to implement persistent objects in Python? (Persistent == automatically saved to and restored from disk.) 4.40. I try to use __spam and I get an error about _SomeClassName__spam. 4.41. How do I delete a file? And other file questions. 4.42. How to modify urllib or httplib to support HTTP/1.1? 4.43. Unexplicable syntax errors in compile() or exec. 4.44. How do I convert a string to a number? 4.45. How do I convert a number to a string? 4.46. How do I copy a file? 4.47. How do I check if an object is an instance of a given class or of a subclass of it? 4.48. What is delegation? 4.49. How do I test a Python program or component. 4.50. My multidimensional list (array) is broken! What gives? 4.51. I want to do a complicated sort: can you do a Schwartzian Transform in Python? 4.52. How to convert between tuples and lists? 4.53. Files retrieved with urllib contain leading garbage that looks like email headers. 4.54. How do I get a list of all instances of a given class? 4.55. A regular expression fails with regex.error: match failure. 4.56. I can't get signal handlers to work. 4.57. I can't use a global variable in a function? Help! 4.58. What's a negative index? Why doesn't list.insert() use them? 4.59. How can I sort one list by values from another list? 4.60. Why doesn't dir() work on builtin types like files and lists? 4.61. How can I mimic CGI form submission (METHOD=POST)? 4.62. If my program crashes with a bsddb (or anydbm) database open, it gets corrupted. How come? 4.63. How do I make a Python script executable on Unix? 4.64. How do you remove duplicates from a list? 4.65. Are there any known year 2000 problems in Python? 4.66. I want a version of map that applies a method to a sequence of objects! Help! 4.67. How do I generate random numbers in Python? 4.68. How do I access the serial (RS232) port? 4.69. Images on Tk-Buttons don't work in Py15? 4.70. Where is the math.py (socket.py, regex.py, etc.) source file? 4.71. How do I send mail from a Python script? 4.72. How do I avoid blocking in connect() of a socket? 4.73. How do I specify hexadecimal and octal integers? 4.74. How to get a single keypress at a time? 4.75. How can I overload constructors (or methods) in Python? 4.76. How do I pass keyword arguments from one method to another? 4.77. What module should I use to help with generating HTML? 4.78. How do I create documentation from doc strings? 4.79. How do I read (or write) binary data? 4.80. I can't get key bindings to work in Tkinter 4.81. "import crypt" fails 4.82. Are there coding standards or a style guide for Python programs? 4.83. How do I freeze Tkinter applications? 4.84. How do I create static class data and static class methods? 4.85. __import__('x.y.z') returns <module 'x'>; how do I get z? 4.86. Basic thread wisdom 4.87. Why doesn't closing sys.stdout (stdin, stderr) really close it? 4.88. What kinds of global value mutation are thread-safe? 4.89. How do I modify a string in place? 4.90. How to pass on keyword/optional parameters/arguments ---------------------------------------------------------------------------- 5. Extending Python 5.1. Can I create my own functions in C? 5.2. Can I create my own functions in C++? 5.3. How can I execute arbitrary Python statements from C? 5.4. How can I evaluate an arbitrary Python expression from C? 5.5. How do I extract C values from a Python object? 5.6. How do I use Py_BuildValue() to create a tuple of arbitrary length? 5.7. How do I call an object's method from C? 5.8. How do I catch the output from PyErr_Print() (or anything that prints to stdout/stderr)? 5.9. How do I access a module written in Python from C? 5.10. How do I interface to C++ objects from Python? 5.11. mSQLmodule (or other old module) won't build with Python 1.5 (or later) 5.12. I added a module using the Setup file and the make fails! Huh? 5.13. I want to compile a Python module on my Red Hat Linux system, but some files are missing. 5.14. What does "SystemError: _PyImport_FixupExtension: module yourmodule not loaded" mean? ---------------------------------------------------------------------------- 6. Python's design 6.1. Why isn't there a switch or case statement in Python? 6.2. Why does Python use indentation for grouping of statements? 6.3. Why are Python strings immutable? 6.4. Why don't strings have methods like index() or sort(), like lists? 6.5. Why does Python use methods for some functionality (e.g. list.index()) but functions for other (e.g. len(list))? 6.6. Why can't I derive a class from built-in types (e.g. lists or files)? 6.7. Why must 'self' be declared and used explicitly in method definitions and calls? 6.8. Can't you emulate threads in the interpreter instead of relying on an OS-specific thread implementation? 6.9. Why can't lambda forms contain statements? 6.10. Why don't lambdas have access to variables defined in the containing scope? 6.11. Why can't recursive functions be defined inside other functions? 6.12. Why is there no more efficient way of iterating over a dictionary than first constructing the list of keys()? 6.13. Can Python be compiled to machine code, C or some other language? 6.14. How does Python manage memory? Why not full garbage collection? 6.15. Why are there separate tuple and list data types? 6.16. How are lists implemented? 6.17. How are dictionaries implemented? 6.18. Why must dictionary keys be immutable? 6.19. How the heck do you make an array in Python? 6.20. Why doesn't list.sort() return the sorted list? 6.21. How do you specify and enforce an interface spec in Python? 6.22. Why do all classes have the same type? Why do instances all have the same type? 6.23. Why isn't all memory freed when Python exits? 6.24. Why no class methods or mutable class variables? 6.25. Why are default values sometimes shared between objects? 6.26. Why no goto? 6.27. How do you make a higher order function in Python? 6.28. Why do I get a SyntaxError for a 'continue' inside a 'try'? 6.29. Why can't raw strings (r-strings) end with a backslash? 6.30. Why can't I use an assignment in an expression? ---------------------------------------------------------------------------- 7. Using Python on non-UNIX platforms 7.1. Is there a Mac version of Python? 7.2. Are there DOS and Windows versions of Python? 7.3. Is there an OS/2 version of Python? 7.4. Is there a VMS version of Python? 7.5. What about IBM mainframes, or other non-UNIX platforms? 7.6. Where are the source or Makefiles for the non-UNIX versions? 7.7. What is the status and support for the non-UNIX versions? 7.8. I have a PC version but it appears to be only a binary. Where's the library? 7.9. Where's the documentation for the Mac or PC version? 7.10. How do I create a Python program file on the Mac or PC? 7.11. How can I use Tkinter on Windows 95/NT? 7.12. cgi.py (or other CGI programming) doesn't work sometimes on NT or win95! 7.13. Why doesn't os.popen() work in PythonWin on NT? 7.14. How do I use different functionality on different platforms with the same program? 7.15. Is there an Amiga version of Python? 7.16. Why doesn't os.popen()/win32pipe.popen() work on Win9x? ---------------------------------------------------------------------------- 8. Python on Windows 8.1. Using Python for CGI on Microsoft Windows 8.2. How to check for a keypress without blocking? 8.3. $PYTHONPATH 8.4. dedent syntax errors 8.5. How do I emulate os.kill() in Windows? 8.6. Why does os.path.isdir() fail on NT shared directories? 8.7. PyRun_SimpleFile() crashes on Windows but not on Unix 8.8. Import of _tkinter fails on Windows 95/98 8.9. Can't extract the downloaded documentation on Windows 8.10. Can't get Py_RunSimpleFile() to work. 8.11. Where is Freeze for Windows? 8.12. Is a *.pyd file the same as a DLL? 8.13. Missing cw3215mt.dll (or missing cw3215.dll) ---------------------------------------------------------------------------- -- ----------- comp.lang.python.announce (moderated) ---------- Article Submission Address: python-announce(a)python.org Python Language Home Page: http://www.python.org/ Python Quick Help Index: http://www.python.org/Help.html ------------------------------------------------------------

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Minotaur 0.1 - bridging Tcl, Python, and Perl
by Jean-Claude Wippler July 7, 1999

July 7, 1999

This is the first public announcement of a generalized extension which lets you run Python scripts from Tcl, or Tcl from Perl, or any of the other combinations. Minotaur works by loading the appropriate shared library into another script language's context. So, for example, running Tcl from Python means that the Python main program imports the Minotaur extension, which in turn loads the Tcl libraries - thus Tcl becomes "embedded" into the Python runtime environment (which is similar to embedding Tcl in a normal C program). As involved as that sounds, performance is already surprisingly good, and the capability is turning out to be quite useful. Minotaur 0.1 is an alpha level release, meaning: it works, aside from a few quirks (see below), and bugs, but its API and implementation might still change in incompatible ways in future releases. An example, embedding a Tcl script in Python: import Minotaur Minotaur.initTcl() print Minotaur.Tcl("expr {1111 + 2222}") Minotaur.Tcl(""" set result "Tcl version [info patchlevel]" Python "result = '$result'" """) print result As you can see, a value is returned either directly (only if it's an integer), or by using a reverse embedding, i.e. executing a Python statement from the embedded Tcl system. Also, this shows how tricky quoting could get once you start mixing languages in this manner. And last but not least, note that the different language contexts are separate: the two "result" variables used above are not related in any way (one for Tcl, one for Python). Also, Minotaur does not yet let you share data, it merely lets you evaluate scripts (and copy things). Now the gotcha's, quite a list of them right now: - You need shared libraries to use Minotaur. Standard installations, especially of Perl and Python, do not always include these. The Minotaur homepage has some info on rebuilds you might need to do. - Tcl/Python can call Perl but to get results back, I had to introduce a secondary shared library, dynamically bound to the Perl runtime. - There is currently no other interface than evaluating a single string. Passing int/string/whatever args efficiently is planned for later. - The Minotaur extension has only been tested for Linux and Windows, but is built in a way which should also work on the Mac, and on all Unix systems, even those which do not support back-linking in shared libs. - Minotaur uses some global data; it's not yet re-entrant / thread-safe. As a matter of fact, Minotaur is still totally ignorant of threads. - I have not yet succeeded in getting Minotaur working on the Mac, and ran into problems when trying to launch Tk from Python (Win/Linux). - The current release does not include any Minotaur binaries, you may have to do quite a bit of tweaking to get this stuff to work for you. One of my plans is to substantially simplify deployment of all this. And here's the fun part of things: - Minotaur uses Forth as low-level "super-glue" language. This offers the ability to deal with all the inevitable "impedance mismatches" which will occur when crossing widely different software designs. Also, this can be done without compilation (and often portably). - Timing tests indicate that the overhead of calling through Minotaur is very low, when the necessary symbol lookups are done in advance. The C-based Forth in here uses pForth, and includes quite a bit of standard ("ANS") Forth for now, though this may change in the future. - The Minotaur extension is loadable from Perl, Python, and Tcl, 'cause it exports initialization calls for each of them. This is essential, since the embedded language needs to reopen the extension to gain access to the same Forth context as the one which loaded it (this is how each embedded language can call back into its parent context). - Forth primitives have been defined which can open, lookup, and locate routines exported from shared libraries. Due to the flexibility and performance of Forth, everything else needed to embed one language in the other is done in Forth, i.e. at run time. Forth has no idea that it is being used to tie scripting languages together, for example. - Minotaur, and these shared library access functions have been tested on Linux, Windows, and Mac (partially). The Forth code needed to implement such language bindings differs only in pathnames, so far. - But this is only the tip of the iceberg, really. Proof-of-concept bindings have also been created for Java, MS COM, Lua, ICI, and PHP. Minotaur is a spinoff of my long-term "TinyScript" project and is open source, with a BSD-style license (do whatever you like with it, just leave the copyright attributions intact, and don't sue me). Minotaur was written to see how far one could go in bringing scripting worlds together, and to help me tie my own extensions into a range of languages without having to write a whole range of interfaces. It hasn't met this goal yet, but my hope is that it will be of use to others and that a number of language experts will extend and improve the mechanism so it works well with all of them. Minotaur has sort of a homepage here: http://mini.net/pub/ts2/minotaur.html A mirror copy of my working area is being tracked here: http://mini.net/pub/ts2/ (a.k.a. ftp://mini.net/pub/ts2/) And a snapshots of the whole area was recently created as: http://mini.net/pub/ts2-19990704.tar.gz There is no documentation, release 0.1 is a "use the source Luke" kind of project, but if you would like to try this out - or better still: participate in whatever way you like - feel free to subscribe to the Minotaur mailing list, by sending an email to minotaur-add(a)mini.net. There are a handful of subscribers now, with just a few outbursts of discussions of where to take this, so far. Which, at this stage, is still a very open-ended question, IMO. Comments, suggestions, praise, but especially offering to help with whetever aspect of Minotaur you'd like to improve are most welcome. Language and platform bashing - yawn - will be redirected to /dev/null. If you wish to help lower language barriers, so that more code can be re-used, more interfacing problems can be solved, and perhaps one day even generic extensions can be written which serve multiple contexts, please have a look at Minotaur and consider helping to make it happen. One interesting option, would be to create a binding in SWIG which generates the low-level glue to Forth, as well as language-specific wrappers. The key advantage is that this could let SWIG be used entirely as a run-time tool, with no compiler/compilation in sight (assuming the target library exists in shared library form, that is). -- Jean-Claude Wippler <jcw(a)equi4.com> P.S. This announcement was posted to a range of newsgroups, but was not crossposted to avoid serving as flame bait for YAFLC (Yet Another Fruitless Language Comparison). The list of newsgroups is: comp.lang.forth comp.lang.misc comp.lang.perl.moderated comp.lang.python comp.lang.tcl It's a lotta posting for such an early project, but that's because there seems to be no established "intra-lingual" newsgroup for this stuff. <P><A HREF="http://mini.net/pub/ts2/minotaur.html">Minotaur 0.1</A> - run Python scripts from Tcl, or Tcl from Perl, or any of the other combinations. (06-Jul-99) -- ----------- comp.lang.python.announce (moderated) ---------- Article Submission Address: python-announce(a)python.org Python Language Home Page: http://www.python.org/ Python Quick Help Index: http://www.python.org/Help.html ------------------------------------------------------------

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New location for whiz, html.py phonebook.pl, and my other goodies
by Neale Pickett July 7, 1999

July 7, 1999

Apparently more people are using my software than I realized :-) The new location for my stuff (short descriptions follow) is: http://starship.python.net/crew/neale/src/ (Which is probably where it should have been in the first place.) For those keeping track, the major offerings are: whiz -- html forms tool for creating "wizards" (anyone want to integrate this into bobo? I hear they're looking for one.) html.py -- some classes for generating HTML, in the spirit of HTMLgen, but all in a single file, and it spits out indented code. phonebook -- HTTP to LDAP gateway; makes a web server look like RFC 1959; also provides nice web output and homepage redirection. See http://ldap.lanl.gov/ for a live demo. other stuff -- IRC class for writing bots and clients, an interactive choose-your-own-adventure CGI, and some other non-pythonic toys. Enjoy Neale Pickett <P><A HREF="http://starship.python.net/crew/neale/src/">Misc. HTML tools (new location)</A> - whiz, for creating HTML "wizards"; html.py, for generating HTML; phonebook, a HTTP to LDAP gateway; IRC bot classes; etc. (07-Jul-99) -- ----------- comp.lang.python.announce (moderated) ---------- Article Submission Address: python-announce(a)python.org Python Language Home Page: http://www.python.org/ Python Quick Help Index: http://www.python.org/Help.html ------------------------------------------------------------

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REGISTRATION REMINDER FOR OPEN SOURCE CONVENTION
by simone＠oreilly.com July 7, 1999

July 7, 1999

If you're planning to attend this year's O'Reilly Open Source Software Convention in Monterey, CA, August 21-24 (covering Perl, Apache, Linux, sendmail, Tcl/Tk, Python, as well as an Open Source business track) , you'll probably want to register soon -- discounts of up to $200, as well as an exclusive Open Source Convention T-shirt, apply ONLY to registrations before July 15th. Hope to see you there! For more information, or to register, see: http://conferences.oreilly.com Simone Paddock O'Reilly & Associates 101 Morris Street Sebastopol, CA 95472 simone(a)oreilly.com -- ----------- comp.lang.python.announce (moderated) ---------- Article Submission Address: python-announce(a)python.org Python Language Home Page: http://www.python.org/ Python Quick Help Index: http://www.python.org/Help.html ------------------------------------------------------------

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Updated Python documentation available!
by Fred L. Drake July 7, 1999

July 7, 1999

Updated documentation for Python 1.5.2 is now available. The online version is available at: http://www.python.org/doc/ Archives of the documentation in HTML, LaTeX, PDF, and PostScript formats are available at the same location, and may also be downloaded via FTP from: ftp://ftp.python.org/pub/python/doc/ If you have any questions or comments on the documentation, please send email to: python-docs(a)python.org Many additional modules have been documented (22 more!). Many small improvements have been made to other parts of the documentation as well. The following modules are now documented: asyncore dl sched chunk fpformat SimpleHTTPServer cmp htmlentitydefs statcache cmpcache mutex statvfs codeop new sunau curses nis SUNAUDIODEV dircache pty tty dl rlcompleter Windows users: If you installed Python using the installer rather than by building from source, you can get the updated documentation by downloading the HTML archive and unpacking it into the "Doc" directory of your installation. Most of the new sections were contributions from the Python community; your help makes the Python documentation some of the best available for any free programming language! Enjoy! -Fred == Fred L. Drake, Jr. <fdrake(a)acm.org> Corporation for National Research Initiatives <P><A HREF="http://www.python.org/doc/">Updated Python 1.5.2 documentation</A> - with 22 additional modules documented and many small improvements. (06-Jul-99) -- ----------- comp.lang.python.announce (moderated) ---------- Article Submission Address: python-announce(a)python.org Python Language Home Page: http://www.python.org/ Python Quick Help Index: http://www.python.org/Help.html ------------------------------------------------------------

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ExpectPy 1.8.2 - Expect extension for Python
by Michael P. Reilly July 7, 1999

July 7, 1999

I've created a patch release for ExpectPy which fixes some Linux bugs and build problems. I have also moved the ExpectPy webpage to Starship Python: http://starship.python.net/crew/arcege/ExpectPy/ The old URL points to the new location. The author's email address is arcege(a)shore.net. Note: Expect does not support Tcl 8.1 (and is expected to support Tcl 8.2 instead); the regular expression engine and interface in Tcl 8.1 changed making Expect and ExpectPy unusable with that release. This may cause problems when compiling ExpectPy with earlier versions of Tcl, and compiling Tkinter with Tcl/Tk 8.1. -Arcege Changes since 1.8: * Linux has different mechanisms for extended CBAUD and for when a terminal detects EOF (the error EIO is returned). * Add helper macros to the code for better readability. * Make the C code more compilable (K&R C). * Fix various build issues. * Add a new option to the configure script to force using static libraries. * Beta releases of Python include a "bN" in the version string, strip this inside the configure script. * Handle searching for the pthread library. * Make some minor changes for using Tcl 8.1 in a later release. * Change the ask.py example to use the new stty method instead of termios (which I find is not compiled on many systems). * Fix a bug where setattr of the settings object could receive a NULL. <P><A HREF="http://starship.python.net/crew/arcege/ExpectPy/">ExpectPy 1.8.2</A> - Python adaptation of Don Libes's "Expect" library for automation of interactive UNIX processes. (05-Jul-99) -- ----------- comp.lang.python.announce (moderated) ---------- Article Submission Address: python-announce(a)python.org Python Language Home Page: http://www.python.org/ Python Quick Help Index: http://www.python.org/Help.html ------------------------------------------------------------

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