Why use def main()? [duplicate]

Without the main sentinel, the code would be executed even if the script were imported as a module.

Everyone else has already answered it, but I think I still have something else to add.

Reasons to have that if statement calling main() (in no particular order):

• Other languages (like C and Java) have a main() function that is called when the program is executed. Using this if, we can make Python behave like them, which feels more familiar for many people.

• Code will be cleaner, easier to read, and better organized. (yeah, I know this is subjective)

• It will be possible to import that python code as a module without nasty side-effects.

• This means it will be possible to run tests against that code.

• This means we can import that code into an interactive python shell and test/debug/run it.

• Variables inside def main are local, while those outside it are global. This may introduce a few bugs and unexpected behaviors.

But, you are not required to write a main() function and call it inside an if statement.

I myself usually start writing small throwaway scripts without any kind of function. If the script grows big enough, or if I feel putting all that code inside a function will benefit me, then I refactor the code and do it. This also happens when I write bash scripts.

Even if you put code inside the main function, you are not required to write it exactly like that. A neat variation could be:

import sys

def main(argv):
    # My code here
    pass

if __name__ == "__main__":
    main(sys.argv)

This means you can call main() from other scripts (or interactive shell) passing custom parameters. This might be useful in unit tests, or when batch-processing. But remember that the code above will require parsing of argv, thus maybe it would be better to use a different call that pass parameters already parsed.

In an object-oriented application I've written, the code looked like this:

class MyApplication(something):
    # My code here

if __name__ == "__main__":
    app = MyApplication()
    app.run()

So, feel free to write the code that better suits you. :)

if the content of foo.py

print __name__
if __name__ == '__main__':
    print 'XXXX'

A file foo.py can be used in two ways.

• imported in another file : import foo

In this case __name__ is foo, the code section does not get executed and does not print XXXX.

• executed directly : python foo.py

When it is executed directly, __name__ is same as __main__ and the code in that section is executed and prints XXXX

One of the use of this functionality to write various kind of unit tests within the same module.