Python-Dev March 2009

python-dev@python.org

173 participants
183 discussions

by Guido van Rossum

I've received some enthusiastic emails from someone who wants to revive restricted mode. He started out with a bunch of patches to the CPython runtime using ctypes, which he attached to an App Engine bug: http://code.google.com/p/googleappengine/issues/detail?id=671 Based on his code (the file secure.py is all you need, included in secure.tar.gz) it seems he believes the only security leaks are __subclasses__, gi_frame and gi_code. (I have since convinced him that if we add "restricted" guards to these attributes, he doesn't need the functions added to sys.) I don't recall the exploits that Samuele once posted that caused the death of rexec.py -- does anyone recall, or have a pointer to the threads? -- --Guido van Rossum (home page: http://www.python.org/~guido/)

5 years, 7 months

pthreads, fork, import, and execvp

by Rotem Yaari

Hello everyone! We have been encountering several deadlocks in a threaded Python application which calls subprocess.Popen (i.e. fork()) in some of its threads. This has occurred on Python 2.4.1 on a 2.4.27 Linux kernel. Preliminary analysis of the hang shows that the child process blocks upon entering the execvp function, in which the import_lock is acquired due to the following line: def _ execvpe(file, args, env=None): from errno import ENOENT, ENOTDIR ... It is known that when forking from a pthreaded application, acquisition attempts on locks which were already locked by other threads while fork() was called will deadlock. Due to these oddities we were wondering if it would be better to extract the above import line from the execvpe call, to prevent lock acquisition attempts in such cases. Another workaround could be re-assigning a new lock to import_lock (such a thing is done with the global interpreter lock) at PyOS_AfterFork or pthread_atfork. We'd appreciate any opinions you might have on the subject. Thanks in advance, Yair and Rotem

10 years, 6 months

compiling python2.5 on linux under wine

by Luke Kenneth Casson Leighton

hey, has anyone investigated compiling python2.5 using winegcc, under wine? i'm presently working my way through it, just for kicks, and was wondering if anyone would like to pitch in or stare at the mess under a microscope. it's not as crazed as it sounds. cross-compiling python2.5 for win32 with mingw32 is an absolute miserable bitch of a job that goes horribly wrong when you actually try to use the minimalist compiler to do any real work. so i figured that it would be easier to get python compiled using wine. i _have_ got some success - a python script and a python.exe.so (which is winegcc's friendly way of telling you you have something that stands a chance of working) as well as a libpython25.dll.so. what i _don't_ yet have is an _md5.dll (or should it be _md5.lib?) i.e. the standard modules are a bit... iffy. the _winreg.o is compiled; the _md5.o is compiled; the winreg.lib is not. whoops. plus, it's necessary to enable nt_dl.c which is in PC/ _not_ in Modules/. one of the key issues that's a bit of a bitch is that python is compiled up for win32 with a hard-coded pyconfig.h which someone went to a _lot_ of trouble to create by hand instead of using autoconf. oh - and it uses visualstudio so there's not even a Makefile. ignoring that for the time-being was what allowed me to get as far as actually having a python interpreter (with no c-based modules). so there's a whole _stack_ of stuff that needs dragging kicking and screaming into the 21st century. there _is_ a reason why i want to do this. actually, there's two. firstly, i sure as shit do _not_ want to buy, download, install _or_ run visual studio. i flat-out refuse to run an MS os and visual studio runs like a dog under wine. secondly, i want a python25.lib which i can use to cross-compile modules for poor windows users _despite_ sticking to my principles and keeping my integrity as a free software developer. thirdly i'd like to cross-compile pywebkitgtk for win32 fourthly i'd like to compile and link applications to the extremely successful and well wicked MSHTML.DLL... in the _wine_ project :) not the one in windows (!) i want to experiment with DOM model manipulation - from python - similar to the OLPC HulaHop project - _but_ i want to compile or cross-compile everything from linux, not windows (see 1 above) fifthly i'd like to see COM (DCOM) working and pywin32 compiled and useable under wine, even if it means having to get a license to use dcom98 and oleauth.lib and oleauth.h etc. and all the developer files needed to link DCOM applications under windows. actually what i'd _really_ like to see is FreeDCE's DCOM work actually damn well finished, it's only been eight years since wez committed the first versions of the IDL and header files, and it's only been over fifteen years since microsoft began its world domination using COM and DCOM. ... but that's another story :) so that's ... five reasons not two. if anyone would like to collaborate on a crazed project with someone who can't count, i'm happy to make available what i've got up to so far, on github.org. l.

10 years, 7 months

GSoC: Core Python development tools

by Daniel (ajax) Diniz

Hi, I'd like to bring up the general idea of using a PSF slot in GSoC2009 to improve the Python development infrastructure. I also happen to have two concrete proposals for that (such a coincidence!). But I assure you the general idea is more important than my proposals :) General: Solving issues that get in core devs' way when they work on Python development could be a nice PSF GSoC project. I believe there are enough code related tasks that would greatly improve Python development workflow but lack manpower to complete. E.g., ISTM that a student working on svnmerge in past SoC editions would've been able to mitigate some painful shortcomings. If the core developers could come up with a list of peeves that would be solvable in code (in existing tools), that would allow for a very useful GSoC proposal. Proposals: These might fit the description above, and they're both tracker related (yep, one-trick-pony here). The upside is that at least one potential mentor is available for each, and I'd be able to offer support to the student. First, the PSF could invest a slot on integrating different tools of the development workflow. Variations of 'file issue at bug tracker, submit code for review' or 'branch locally to virtualenv, upload failing testcase to tracker, upload patch to tracker' command line utils. If these could be developed as general tools (i.e., not deeply tied to Python dev infrastructure), Ali Afshar from PIDA is willing to mentor it. I'd be available to help with Roundup and Rietveld (integration in progress), but would like to see it work with Launchpad, Bugzilla, Google Code and Review Board :) The other proposal is to use a slot in Roundup proper and the Python tracker instance. This could have a core Roundup developer as mentor, under the condition it's focused on Roundup itself. IMO, are some missing/broken core features in Roundup that would have a positive impact on our tracker, mostly those relating to searches, security and UI. I'd have a lot to contribute to the Python tracker part, based on ongoing work. Even if neither is considered worthy, I'll keep them on my to-do list and hope to slowly and hackishly work towards both proposals' goals. Barring feedback saying that they're out of scope, stupid and downright offensive, I'll post details on each to this thread very soon. So, if the PSF was to use a slot on improving the way you work on Python development, what would you like to see fixed or implemented? Best regards, Daniel

10 years, 8 months

Re: subprocess and EINTR errnos

by Peter Astrand

On Wed, 10 Nov 2004, John P Speno wrote: Hi, sorry for the delayed response. > While using subprocess (aka popen5), I came across one potential gotcha. I've had > exceptions ending like this: > > File "test.py", line 5, in test > cmd = popen5.Popen(args, stdout=PIPE) > File "popen5.py", line 577, in __init__ > data = os.read(errpipe_read, 1048576) # Exceptions limited to 1 MB > OSError: [Errno 4] Interrupted system call > > (on Solaris 9) > > Would it make sense for subprocess to use a more robust read() function > which can handle these cases, i.e. when the parent's read on the pipe > to the child's stderr is interrupted by a system call, and returns EINTR? > I imagine it could catch EINTR and EAGAIN and retry the failed read(). I assume you are using signals in your application? The os.read above is not the only system call that can fail with EINTR. subprocess.py is full of other system calls that can fail, and I suspect that many other Python modules are as well. I've made a patch (attached) to subprocess.py (and test_subprocess.py) that should guard against EINTR, but I haven't committed it yet. It's quite large. Are Python modules supposed to handle EINTR? Why not let the C code handle this? Or, perhaps the signal module should provide a sigaction function, so that users can use SA_RESTART. Index: subprocess.py =================================================================== RCS file: /cvsroot/python/python/dist/src/Lib/subprocess.py,v retrieving revision 1.8 diff -u -r1.8 subprocess.py --- subprocess.py 7 Nov 2004 14:30:34 -0000 1.8 +++ subprocess.py 17 Nov 2004 19:42:30 -0000 @@ -888,6 +888,50 @@ pass + def _read_no_intr(self, fd, buffersize): + """Like os.read, but retries on EINTR""" + while True: + try: + return os.read(fd, buffersize) + except OSError, e: + if e.errno == errno.EINTR: + continue + else: + raise + + + def _read_all(self, fd, buffersize): + """Like os.read, but retries on EINTR, and reads until EOF""" + all = "" + while True: + data = self._read_no_intr(fd, buffersize) + all += data + if data == "": + return all + + + def _write_no_intr(self, fd, s): + """Like os.write, but retries on EINTR""" + while True: + try: + return os.write(fd, s) + except OSError, e: + if e.errno == errno.EINTR: + continue + else: + raise + + def _waitpid_no_intr(self, pid, options): + """Like os.waitpid, but retries on EINTR""" + while True: + try: + return os.waitpid(pid, options) + except OSError, e: + if e.errno == errno.EINTR: + continue + else: + raise + def _execute_child(self, args, executable, preexec_fn, close_fds, cwd, env, universal_newlines, startupinfo, creationflags, shell, @@ -963,7 +1007,7 @@ exc_value, tb) exc_value.child_traceback = ''.join(exc_lines) - os.write(errpipe_write, pickle.dumps(exc_value)) + self._write_no_intr(errpipe_write, pickle.dumps(exc_value)) # This exitcode won't be reported to applications, so it # really doesn't matter what we return. @@ -979,7 +1023,7 @@ os.close(errwrite) # Wait for exec to fail or succeed; possibly raising exception - data = os.read(errpipe_read, 1048576) # Exceptions limited to 1 MB + data = self._read_all(errpipe_read, 1048576) # Exceptions limited to 1 MB os.close(errpipe_read) if data != "": child_exception = pickle.loads(data) @@ -1003,7 +1047,7 @@ attribute.""" if self.returncode == None: try: - pid, sts = os.waitpid(self.pid, os.WNOHANG) + pid, sts = self._waitpid_no_intr(self.pid, os.WNOHANG) if pid == self.pid: self._handle_exitstatus(sts) except os.error: @@ -1015,7 +1059,7 @@ """Wait for child process to terminate. Returns returncode attribute.""" if self.returncode == None: - pid, sts = os.waitpid(self.pid, 0) + pid, sts = self._waitpid_no_intr(self.pid, 0) self._handle_exitstatus(sts) return self.returncode @@ -1049,27 +1093,33 @@ stderr = [] while read_set or write_set: - rlist, wlist, xlist = select.select(read_set, write_set, []) + try: + rlist, wlist, xlist = select.select(read_set, write_set, []) + except select.error, e: + if e[0] == errno.EINTR: + continue + else: + raise if self.stdin in wlist: # When select has indicated that the file is writable, # we can write up to PIPE_BUF bytes without risk # blocking. POSIX defines PIPE_BUF >= 512 - bytes_written = os.write(self.stdin.fileno(), input[:512]) + bytes_written = self._write_no_intr(self.stdin.fileno(), input[:512]) input = input[bytes_written:] if not input: self.stdin.close() write_set.remove(self.stdin) if self.stdout in rlist: - data = os.read(self.stdout.fileno(), 1024) + data = self._read_no_intr(self.stdout.fileno(), 1024) if data == "": self.stdout.close() read_set.remove(self.stdout) stdout.append(data) if self.stderr in rlist: - data = os.read(self.stderr.fileno(), 1024) + data = self._read_no_intr(self.stderr.fileno(), 1024) if data == "": self.stderr.close() read_set.remove(self.stderr) Index: test/test_subprocess.py =================================================================== RCS file: /cvsroot/python/python/dist/src/Lib/test/test_subprocess.py,v retrieving revision 1.14 diff -u -r1.14 test_subprocess.py --- test/test_subprocess.py 12 Nov 2004 15:51:48 -0000 1.14 +++ test/test_subprocess.py 17 Nov 2004 19:42:30 -0000 @@ -7,6 +7,7 @@ import tempfile import time import re +import errno mswindows = (sys.platform == "win32") @@ -35,6 +36,16 @@ fname = tempfile.mktemp() return os.open(fname, os.O_RDWR|os.O_CREAT), fname + def read_no_intr(self, obj): + while True: + try: + return obj.read() + except IOError, e: + if e.errno == errno.EINTR: + continue + else: + raise + # # Generic tests # @@ -123,7 +134,7 @@ p = subprocess.Popen([sys.executable, "-c", 'import sys; sys.stdout.write("orange")'], stdout=subprocess.PIPE) - self.assertEqual(p.stdout.read(), "orange") + self.assertEqual(self.read_no_intr(p.stdout), "orange") def test_stdout_filedes(self): # stdout is set to open file descriptor @@ -151,7 +162,7 @@ p = subprocess.Popen([sys.executable, "-c", 'import sys; sys.stderr.write("strawberry")'], stderr=subprocess.PIPE) - self.assertEqual(remove_stderr_debug_decorations(p.stderr.read()), + self.assertEqual(remove_stderr_debug_decorations(self.read_no_intr(p.stderr)), "strawberry") def test_stderr_filedes(self): @@ -186,7 +197,7 @@ 'sys.stderr.write("orange")'], stdout=subprocess.PIPE, stderr=subprocess.STDOUT) - output = p.stdout.read() + output = self.read_no_intr(p.stdout) stripped = remove_stderr_debug_decorations(output) self.assertEqual(stripped, "appleorange") @@ -220,7 +231,7 @@ stdout=subprocess.PIPE, cwd=tmpdir) normcase = os.path.normcase - self.assertEqual(normcase(p.stdout.read()), normcase(tmpdir)) + self.assertEqual(normcase(self.read_no_intr(p.stdout)), normcase(tmpdir)) def test_env(self): newenv = os.environ.copy() @@ -230,7 +241,7 @@ 'sys.stdout.write(os.getenv("FRUIT"))'], stdout=subprocess.PIPE, env=newenv) - self.assertEqual(p.stdout.read(), "orange") + self.assertEqual(self.read_no_intr(p.stdout), "orange") def test_communicate(self): p = subprocess.Popen([sys.executable, "-c", @@ -305,7 +316,8 @@ 'sys.stdout.write("\\nline6");'], stdout=subprocess.PIPE, universal_newlines=1) - stdout = p.stdout.read() + + stdout = self.read_no_intr(p.stdout) if hasattr(open, 'newlines'): # Interpreter with universal newline support self.assertEqual(stdout, @@ -343,7 +355,7 @@ def test_no_leaking(self): # Make sure we leak no resources - max_handles = 1026 # too much for most UNIX systems + max_handles = 10 # too much for most UNIX systems if mswindows: max_handles = 65 # a full test is too slow on Windows for i in range(max_handles): @@ -424,7 +436,7 @@ 'sys.stdout.write(os.getenv("FRUIT"))'], stdout=subprocess.PIPE, preexec_fn=lambda: os.putenv("FRUIT", "apple")) - self.assertEqual(p.stdout.read(), "apple") + self.assertEqual(self.read_no_intr(p.stdout), "apple") def test_args_string(self): # args is a string @@ -457,7 +469,7 @@ p = subprocess.Popen(["echo $FRUIT"], shell=1, stdout=subprocess.PIPE, env=newenv) - self.assertEqual(p.stdout.read().strip(), "apple") + self.assertEqual(self.read_no_intr(p.stdout).strip(), "apple") def test_shell_string(self): # Run command through the shell (string) @@ -466,7 +478,7 @@ p = subprocess.Popen("echo $FRUIT", shell=1, stdout=subprocess.PIPE, env=newenv) - self.assertEqual(p.stdout.read().strip(), "apple") + self.assertEqual(self.read_no_intr(p.stdout).strip(), "apple") def test_call_string(self): # call() function with string argument on UNIX @@ -525,7 +537,7 @@ p = subprocess.Popen(["set"], shell=1, stdout=subprocess.PIPE, env=newenv) - self.assertNotEqual(p.stdout.read().find("physalis"), -1) + self.assertNotEqual(self.read_no_intr(p.stdout).find("physalis"), -1) def test_shell_string(self): # Run command through the shell (string) @@ -534,7 +546,7 @@ p = subprocess.Popen("set", shell=1, stdout=subprocess.PIPE, env=newenv) - self.assertNotEqual(p.stdout.read().find("physalis"), -1) + self.assertNotEqual(self.read_no_intr(p.stdout).find("physalis"), -1) def test_call_string(self): # call() function with string argument on Windows /Peter Åstrand <astrand(a)lysator.liu.se>

10 years, 9 months

Evaluated cmake as an autoconf replacement

by Jeffrey Yasskin

I've heard some good things about cmake — LLVM, googletest, and Boost are all looking at switching to it — so I wanted to see if we could simplify our autoconf+makefile system by using it. The biggest wins I see from going to cmake are: 1. It can autogenerate the Visual Studio project files instead of needing them to be maintained separately 2. It lets you write functions and modules without understanding autoconf's mix of shell and M4. 3. Its generated Makefiles track header dependencies accurately so we might be able to add private headers efficiently. However, after trying it out during PyCon and looking over the previous attempt at <http://mail.python.org/pipermail/python-dev/2007-July/073912.html>, I can't recommend switching to it. A. It has no equivalent of autoheader, so we'd have to maintain pyconfig.h.in by hand. B. It does not allow the CMakeLists.txt file control the --help output. This appears to be an intentional decision (http://www.cmake.org/pipermail/cmake-promote/2006-May/000095.html). To replace it, they have an interactive mode (which asks you about each possible option) and a -LH flag (which runs the whole configure process and then tells you about the flags you could have set if you knew about them). C. It does not allow the CMakeLists.txt file to see the command line, so we can't stay backward compatible with the configure script, and we'd have to replace flags like --with-pydebug with -Dpydebug:bool=true. We could write a separate configure script to mitigate this and the previous problem, but we'd have to keep that in sync with CMakeLists.txt manually. D. It doesn't have an expression language, so to replace ac_md_system=`echo $ac_sys_system | tr -d '[/ ]' | tr '[[A-Z]]' '[[a-z]]'` you have to write: string(REGEX REPLACE "[/ ]" "" ac_md_system ${ac_sys_system}) string(TOLOWER ${ac_md_system} ac_md_system) So, at the very least, it doesn't look like a big enough win to justify switching, and may not be a win at all. The other popular configure+make replacement is scons. The biggest objection to scons before even looking at it is that it requires a working Python interpreter in order to build Python, causing a bootstrap problem. However, Brett Cannon and I talked, and we think this is surmountable. First, nearly every desktop system comes with a Python interpreter, which would avoid the bootstrap for ordinary development. Second, we need a cross compilation story anyway for embedded systems, and the same mechanism could be used to get Python onto a new desktop platform. Third, Jython and IronPython are pretty mature and either can run scons now or should be able to run it after some relatively easy tweaks. They could be used to build CPython on a new platform. I don't intend to look into scons myself any time soon, but I'd be interested in anyone's experiences who does try it. Jeffrey

10 years, 11 months

pyc files, constant folding and borderline portability issues

by Antoine Pitrou

Hello, There are a couple of ancillary portability concerns due to optimizations which store system-dependent results of operations between constants in pyc files: - Issue #5057: code like '\U00012345'[0] is optimized away and its result stored as a constant in the pyc file, but the result should be different in UCS-2 and UCS-4 builds. - Issue #5593: code like 1e16+2.9999 is optimized away and its result stored as a constant (again), but the result can vary slightly depending on the internal FPU precision. These problems have probably been there for a long time and almost no one seems to complain, but I thought I'd report them here just in case. Regards Antoine.

10 years, 12 months

os.defpath for Windows

by Yinon Ehrlich

Hi, just saw that os.defpath for Windows is defined as Lib/ntpath.py:30:defpath = '.;C:\\bin' Most Windows machines I saw has no c:\bin directory. Any reason why it was defined this way ? Thanks, Yinon

10 years, 12 months

deprecating BaseException.message

by Brett Cannon

During the PyCon sprint I tried to make BaseException accept only a single argument and bind it to BaseException.message . I was successful (see the p3yk_no_args_on_exc branch), but it was very painful to pull off as anyone who sat around me the last three days of the sprint will tell you as they had to listen to me curse incessantly. Because of the pain that I went through in the transition and thus the lessons learned, Guido and I discussed it and we think it would be best to give up on forcing BaseException to accept only a single argument. I think it is still doable, but requires a multi-release transition period and not the one that 2.6 -> 3.0 is offering. And so Guido and I plan on deprecating BaseException.message as its entire point in existence was to help transition to what we are not going to have happen. =) Now that means BaseException.message might hold the record for shortest lived feature as it was only introduced in 2.5 and is now to be deprecated in 2.6 and removed in 2.7/3.0. =) Below is PEP 352, revised to reflect the removal of BaseException.messageand for letting the 'args' attribute stay (along with suggesting one should only pass a single argument to BaseException). Basically the interface for exceptions doesn't really change in 3.0 except for the removal of __getitem__. -------------------------------------------------------------------------- PEP: 352 Title: Required Superclass for Exceptions Version: $Revision: 53592 $ Last-Modified: $Date: 2007-01-28 21:54:11 -0800 (Sun, 28 Jan 2007) $ Author: Brett Cannon <brett(a)python.org> Guido van Rossum <guido(a)python.org> Status: Final Type: Standards Track Content-Type: text/x-rst Created: 27-Oct-2005 Post-History: Abstract ======== In Python 2.4 and before, any (classic) class can be raised as an exception. The plan for 2.5 was to allow new-style classes, but this makes the problem worse -- it would mean *any* class (or instance) can be raised! This is a problem as it prevents any guarantees from being made about the interface of exceptions. This PEP proposes introducing a new superclass that all raised objects must inherit from. Imposing the restriction will allow a standard interface for exceptions to exist that can be relied upon. It also leads to a known hierarchy for all exceptions to adhere to. One might counter that requiring a specific base class for a particular interface is unPythonic. However, in the specific case of exceptions there's a good reason (which has generally been agreed to on python-dev): requiring hierarchy helps code that wants to *catch* exceptions by making it possible to catch *all* exceptions explicitly by writing ``except BaseException:`` instead of ``except *:``. [#hierarchy-good]_ Introducing a new superclass for exceptions also gives us the chance to rearrange the exception hierarchy slightly for the better. As it currently stands, all exceptions in the built-in namespace inherit from Exception. This is a problem since this includes two exceptions (KeyboardInterrupt and SystemExit) that often need to be excepted from the application's exception handling: the default behavior of shutting the interpreter down without a traceback is usually more desirable than whatever the application might do (with the possible exception of applications that emulate Python's interactive command loop with ``>>>`` prompt). Changing it so that these two exceptions inherit from the common superclass instead of Exception will make it easy for people to write ``except`` clauses that are not overreaching and not catch exceptions that should propagate up. This PEP is based on previous work done for PEP 348 [#pep348]_. Requiring a Common Superclass ============================= This PEP proposes introducing a new exception named BaseException that is a new-style class and has a single attribute, ``args``. Below is the code as the exception will work in Python 3.0 (how it will work in Python 2.x is covered in the `Transition Plan`_ section):: class BaseException(object): """Superclass representing the base of the exception hierarchy. Provides a 'message' attribute that contains either the single argument to the constructor or the empty string. This attribute is used in the string representation for the exception. This is so that it provides the extra details in the traceback. """ def __init__(self, *args): """Set the 'message' attribute'""" self.args = args def __str__(self): """Return the str of 'message'""" if len(self.args) == 1: return str(self.args[0]) else: return str(self.args) def __repr__(self): return "%s(*%s)" % (self.__class__.__name__, repr(self.args)) No restriction is placed upon what may be passed in for ``args`` for backwards-compatibility reasons. In practice, though, only a single string argument should be used. This keeps the string representation of the exception to be a useful message about the exception that is human-readable; this is why the ``__str__`` method special-cases on length-1 ``args`` value. Including programmatic information (e.g., an error code number) should be stored as a separate attribute in a subclass. The ``raise`` statement will be changed to require that any object passed to it must inherit from BaseException. This will make sure that all exceptions fall within a single hierarchy that is anchored at BaseException [#hierarchy-good]_. This also guarantees a basic interface that is inherited from BaseException. The change to ``raise`` will be enforced starting in Python 3.0 (see the `Transition Plan`_ below). With BaseException being the root of the exception hierarchy, Exception will now inherit from it. Exception Hierarchy Changes =========================== With the exception hierarchy now even more important since it has a basic root, a change to the existing hierarchy is called for. As it stands now, if one wants to catch all exceptions that signal an error *and* do not mean the interpreter should be allowed to exit, you must specify all but two exceptions specifically in an ``except`` clause or catch the two exceptions separately and then re-raise them and have all other exceptions fall through to a bare ``except`` clause:: except (KeyboardInterrupt, SystemExit): raise except: ... That is needlessly explicit. This PEP proposes moving KeyboardInterrupt and SystemExit to inherit directly from BaseException. :: - BaseException |- KeyboardInterrupt |- SystemExit |- Exception |- (all other current built-in exceptions) Doing this makes catching Exception more reasonable. It would catch only exceptions that signify errors. Exceptions that signal that the interpreter should exit will not be caught and thus be allowed to propagate up and allow the interpreter to terminate. KeyboardInterrupt has been moved since users typically expect an application to exit when the press the interrupt key (usually Ctrl-C). If people have overly broad ``except`` clauses the expected behaviour does not occur. SystemExit has been moved for similar reasons. Since the exception is raised when ``sys.exit()`` is called the interpreter should normally be allowed to terminate. Unfortunately overly broad ``except`` clauses can prevent the explicitly requested exit from occurring. To make sure that people catch Exception most of the time, various parts of the documentation and tutorials will need to be updated to strongly suggest that Exception be what programmers want to use. Bare ``except`` clauses or catching BaseException directly should be discouraged based on the fact that KeyboardInterrupt and SystemExit almost always should be allowed to propagate up. Transition Plan =============== Since semantic changes to Python are being proposed, a transition plan is needed. The goal is to end up with the new semantics being used in Python 3.0 while providing a smooth transition for 2.x code. All deprecations mentioned in the plan will lead to the removal of the semantics starting in the version following the initial deprecation. Here is BaseException as implemented in the 2.x series:: class BaseException(object): """Superclass representing the base of the exception hierarchy. The __getitem__ method is provided for backwards-compatibility and will be deprecated at some point. """ def __init__(self, *args): """Set the 'args' attribute.""" self.args = args def __str__(self): """Return the str of args[0] or args, depending on length.""" return str(self.args[0] if len(self.args) <= 1 else self.args) def __repr__(self): func_args = repr(self.args) if self.args else "()" return self.__class__.__name__ + func_args def __getitem__(self, index): """Index into arguments passed in during instantiation. Provided for backwards-compatibility and will be deprecated. """ return self.args[index] Deprecation of features in Python 2.9 is optional. This is because it is not known at this time if Python 2.9 (which is slated to be the last version in the 2.x series) will actively deprecate features that will not be in 3.0 . It is conceivable that no deprecation warnings will be used in 2.9 since there could be such a difference between 2.9 and 3.0 that it would make 2.9 too "noisy" in terms of warnings. Thus the proposed deprecation warnings for Python 2.9 will be revisited when development of that version begins to determine if they are still desired. * Python 2.5 [done] - all standard exceptions become new-style classes - introduce BaseException - Exception, KeyboardInterrupt, and SystemExit inherit from BaseException - deprecate raising string exceptions * Python 2.6 - deprecate catching string exceptions - deprecate ``message`` attribute (see `Retracted Ideas`_) * Python 2.7 - deprecate raising exceptions that do not inherit from BaseException - remove ``message`` attribute * Python 2.8 - deprecate catching exceptions that do not inherit from BaseException * Python 2.9 - deprecate ``__getitem__`` (optional) * Python 3.0 [done] - drop everything that was deprecated above: + string exceptions (both raising and catching) + all exceptions must inherit from BaseException + drop ``__getitem__`` Retracted Ideas =============== A previous version of this PEP that was implemented in Python 2.5 included a 'message' attribute on BaseException. Its purpose was to begin a transition to BaseException accepting only a single argument. This was to tighten the interface and to force people to use attributes in subclasses to carry arbitrary information with an exception instead of cramming it all into ``args``. Unfortunately, while implementing the removal of the ``args`` attribute in Python 3.0 at the PyCon 2007 sprint [#pycon2007-sprint-email]_, it was discovered that the transition was very painful, especially for C extension modules. It was decided that it would be better to deprecate the ``message`` attribute in Python 2.6 (and remove in Python 2.7 and Python 3.0) and consider a more long-term transition strategy in Python 3.0 to remove multiple-argument support in BaseException in preference of accepting only a single argument. Thus the introduction of ``message`` and the original deprecation of ``args`` has been retracted. References ========== .. [#pep348] PEP 348 (Exception Reorganization for Python 3.0) http://www.python.org/peps/pep-0348.html .. [#hierarchy-good] python-dev Summary for 2004-08-01 through 2004-08-15 http://www.python.org/dev/summary/2004-08-01_2004-08-15.html#an-exception-i… .. [#SF_1104669] SF patch #1104669 (new-style exceptions) http://www.python.org/sf/1104669 .. [#pycon2007-sprint-email] python-3000 email ("How far to go with cleaning up exceptions") http://mail.python.org/pipermail/python-3000/2007-March/005911.html Copyright ========= This document has been placed in the public domain. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 End:

10 years, 12 months

Re: [Python-Dev] Integrate BeautifulSoup into stdlib?

by Jim Jewett

Michael Foord wrote: > Chris Withers wrote: >> ... love to see ... but ... not optimistic >> - python to grow a decent, cross platform, package management system As stated, this may be impossible, because of the difference in what a package should mean on Windows vs Unix. If you just mean a way to add python packages from pypi as with EasyInstall, then maybe. >> - the standard library to actually shrink to a point where only >> libraries that are not released elsewhere are included In some environments, each new component must be approved. Once python is approved, the standard library is OK, but adding 7 packages from pypi requires 7 more sets of approvals. On the other hand, if there were a way to say "The PSF explicitly endorses Packages X, Y, and Z as worthy of the stdlib; they are distributed separately for administrative reasons", then the initial request could be for "Python plus officially endorsed addons" That said, it may make sense to just give greater prominence to existing repackagers, such as ActiveState or Enthought. > If a library is well maintained then there seems to be little point in > moving it into the standard library The official endorsement is in many cases more important than shared distribution. -jJ

11 years

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Python-Dev March 2009