[Tutor] A few comparative perl/python questions

[Danny Yoo]

On Sun, 28 Jul 2002, Kyle Babich wrote:

> I just have a few Perl-to-Python questions:
>
> Does python have anything like Perl's || die, flock(), or seek()
> functions?  (What are they?)

[Aside on Perl]

Perl has pretty nice documentation in the 'perldoc' utility: here's an
example of finding out information on the die() function:

###
dyoo@coffeetable:~$ perldoc -f die
die LIST
           Outside an "eval", prints the value of LIST to "STDERR" and
           exits with the current value of "$!" (errno).  If "$!" is "0",
           exits with the value of "($? >> 8)" (backtick `command` sta-
           tus).  If "($? >> 8)" is "0", exits with "255".  Inside an
           "eval()," the error message is stuffed into "$@" and the "eval"
           is terminated with the undefined value.  This makes "die" the
           way to raise an exception.
###

So if you're working with perl, 'perldoc' is your friend.  In Python, we
have the help() function, which does similar things:

###
>>> help(file.seek)
Help on method_descriptor:

seek(...)
    seek(offset[, whence]) -> None.  Move to new file position.

    Argument offset is a byte count.  Optional argument whence defaults to
    0 (offset from start of file, offset should be >= 0); other values are
    1 (move relative to current position, positive or negative), and 2
    (move relative to end of file, usually negative, although many platforms
    allow seeking beyond the end of a file).

    Note that not all file objects are seekable.
###


Simply speaking, die() causes the program to exit immediately with an
error message.  This is useful if something "bad" happens.  "Bad"  things
might include things like:

    Trying to open a file that doesn't exist.
    Trying to connect to a database with an incorrect password.
    Trying to divide by zero.


In Python, an equivalent way to make a program die is to "raise" an
exception, like this:

###
>>> def mydivide(a, b):
...     if b == 0:
...         raise ZeroDivisionError, "You bad person!"
...     return a / b
...
>>>
>>> mydivide(12, 4)
3
>>> mydivide(12, 0)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 3, in mydivide
ZeroDivisionError: You bad person!
###



But actually, division raises a ZeroDivisionError anyway:

###
>>> 12/0
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ZeroDivisionError: integer division or modulo by zero
###

so the example above is sorta Useless.  Still, it shows how we might be
able to make a program try to die quickly if we believe it should.  If
you're interested in exceptions, you may find:

    http://www.python.org/doc/tut/node10.html

a "deep-end-of-the-pool" introduction to exception handling.

Alan Gauld's "Learning to Program" has a much gentler introduction to
error handling here:

    http://www.freenetpages.co.uk/hp/alan.gauld/tuterrors.htm



> If Python doesn't have something like flock(), what happens what
> multiple scripts are trying to write to one file at the same time?

Python does have flock(), but it's in the somewhat unpronouncable 'fcntl'
"file control" module:

    http://www.python.org/doc/lib/module-fcntl.html

The only problem is that this appears to be Unix-specific.  Hmmm... Ah!
The Python Cookbook includes a module to make this file locking work on
Win32 platforms too:

    http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/65203


Hope this helps!