[Python-ideas] PEP for executing a module in a package containing relative imports

Neil Toronto ntoronto at cs.byu.edu
Mon Apr 23 01:51:26 CEST 2007

Steven Bethard wrote:
> On 4/22/07, Jim Jewett <jimjjewett at gmail.com> wrote:
>> # Equivalent to today
>> if __name__ == sys.modules["__main__"].__name__:
>> # Better than today
>> if __name__ is sys.modules["__main__"].__name__:
>> # What I would like (pending PEP I hope to write tonight)
>> if __this_module__ is sys.modules["__main__"]:
> Is it just me, or are the proposals starting to look more and more like::
>     public static void main(String args[])
> I think this PEP now needs to explicitly state that keeping the "am I
> the main module?" idiom as simple as possible is *not* a goal. Because
> everything I've seen (except for the original proposals in the PEP)
> are substantially more complicated than the current::
>     if __name__ == '__main__':
> I guess I don't understand why we wouldn't be willing to put up with a
> new module attribute or builtin to minimize the boilerplate in pretty
> much every Python application out there.

Agreed - it's getting horrid. As Pythonic as they think this is, they're 
completely forgetting the newb.

So let's look at it from his point of view. Say I'm a Python newb. I've 
written some modules and some executable Python scripts and I'm somewhat 
comfy with the language. (Of course, it only took me about two hours to 
get comfy - this is Python, after all.) I now want to write either:

1) A module that runs unit tests when it is run as a script, but not 
when it's just imported; or
2) A script that can be imported as a module when I need a few of its 
functions. (I should really split them into another module, but this is 
a use case.)

Now I have to import sys? Never seen that one... okay. Imported. Now, 
what's this Greek I have to write to test whether the script is the main 
script? How am I supposed to remember this? This is worse than fork()!

On the other hand, IMNSHO, either of the following two are just about 
perfect in terms of understandability, and parsimony:

    def __main__():  # we really don't need args here
        # stuff

    if __main__:
        # stuff

Chances are, the first will be very familiar, but refreshing that it's 
just a plain old, gibberish-free function. Both are easier than what 
we've got currently. (IMO, the first is better, because 1) the code can 
be put anywhere in the module; 2) it automatically doesn't pollute the 
global namespace; and 3) it's less boilerplate for complex modules and 
no more boilerplate for simple ones.)

FWIW, I don't see a problem with a sys.modules['__main__'] - it would 
even occasionally be useful - but nobody should be *required* to use an 
abomination like that for what's clearly a newbie task: determining 
whether a module is run as a script.


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