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[Python-Dev] PEP 395: Module Aliasing

Nick Coghlan ncoghlan at gmail.com
Sat Mar 5 01:38:38 CET 2011 
On Sat, Mar 5, 2011 at 2:22 AM, Fred Drake <fdrake at acm.org> wrote:
> On Fri, Mar 4, 2011 at 10:59 AM,  <exarkun at twistedmatrix.com> wrote:
>> Something to consider here is how this will interact with Python files which
>> are _not_ modules.  I'm a little uneasy about having sys.modules["trial"]
>> refer to the module defined by /usr/bin/trial.
>
> I've long held the position that modules and scripts are distinct, and
> should never share a source file.  All the work that's going into
> dealing with this just reinforces my position.

While I actually have a lot of respect for that point of view (the
traps in PEP 395 can be read as a list of justifications for it!),
"python -m timeit", "python -m test", "python -m site" and assorted
other instances in the stdlib disagree with it. I don't think a purity
argument is going to fly here, because the practice of allowing
executable modules is just so damn useful for utility functionality
where a developer doesn't want to worry about global namespace
collisions on the system path . If we turned every module we have in
the stdlib with a little snippet of directly executable functionality
at the bottom into an installed script, I assume the distro packagers
would either outright ignore us or howl the place down with cries of
namespace pollution (and they'd be right to do so - the "-m" switch
*originated* in a discussion about making timeit easier to invoke on
different parallel Python installations without running into namespace
collision problems on the system path).

Besides, this particular ship really sailed way back in the early days
of Python's history when the "if __name__ == '__main__':" convention
was established to allow the same source file to behave differently
depending on how it was invoked (either as a module or as a script).
All PEP 395 is about is trying to make the way that works line up more
closely with people's intuitions.

However, as a slight tangent, note that one of the positive side
effects of the __main__.py file approach to directory and package
execution is that it *does* promote keeping the script behaviour and
the module behaviour in separate files.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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