[Python-ideas] Replacing the if __name__ == "__main__" idiom (was Re: making a module callable)

[Markus Unterwaditzer]

+1 for the second approach, because

1.) It'd be more familiar to programmers coming from C, Java, etc. But I don't see how it would be easier to learn to completely new programmers.

2.) Forces people not to pollute the global namespace by making them write their code inside a function. That's usually a good idea anyways, which is why people already do

if __name__ == "__main__":
main()

-- Markus

Steven D'Aprano <steve at pearwood.info> wrote:
>On Fri, Nov 22, 2013 at 05:45:22PM -0800, Sean McQuillan wrote:
>> Adding a mainfunction decorator was mentioned in the other thread,
>seemed
>> interesting so I coded up a simple example to see how it works.
>> 
>>
>https://github.com/objcode/mainfunction/blob/master/mainfunction/mainfunction.py
>
>I haven't actually tried it, but by reading the code I don't think that
>
>works the way I expect a main function to work. Your decorator simply 
>does a bit of book-keeping, then immediately calls the main function.
>So 
>it doesn't so much define a main function as just execute it straight 
>away, which means it works with simple cases like this:
>
>
>@mainfunction
>def main():
>    print "Hello, World."
>
>
>but not for cases like this:
>
>
>@mainfunction
>def main():
>    do_stuff("Hello, World!")
>
>def do_stuff(msg):
>    print msg
>
>
>since do_stuff doesn't exist at the time main() is executed. In 
>contrast, the standard idiom is typically the very last thing at the 
>bottom of the file, and so by the time it is called all the support 
>functions are in place.
>
>My aesthetic sense tells me that there are two reasonable approaches 
>here:
>
>1) The current idiom, with an explicit "if __name__" test. This is the 
>most flexible.
>
>2) An implicit main function, which I think should be spelled __main__ 
>to emphasise that it is special. This, I think, would require support 
>from the compiler. (At least, I can't see any easy way to bootstrap it 
>without using the "if __name__" test above.)
>
>python some_file.py
>
>python -m some_file
>
>would both execute the file as normal. The only addition would be,
>after 
>the file has been run the compiler checks whether there is a name 
>"__main__" in the global namespace, and if so, calls that __main__ 
>object with sys.argv as argument.
>
>This second is analogous to the situation with packages. If a package 
>has a __main__.py file, it is run when you call
>
>python -m package
>
>
>This second approach is purely a convenience for writing scripts, it 
>doesn't given you anything you can't already do with the current idiom,
>
>but I think that defining a special main function
>
>def __main__(argv):
>    ...
>
>
>is a cleaner (but less flexible) idiom that the current if __name__ 
>business, and simpler for people to learn.
>
>
>-- 
>Steven
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