Why use def main()?

Possible Duplicate:
What does if __name__== "__main__" do?

I've seen some code samples and tutorials that use

def main():
    # my code here

if __name__ == "__main__":
    main()

But why? Is there any reason not do define your functions at the top of the file, then just write code under it? ie

def my_function()
    # my code here

def my_function_two()
    # my code here

# some code
# call function
# print(something)

I just wonder if there is any rhyme to the main?


如果没有主标记,即使脚本作为模块导入,代码也会执行。


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 as 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.

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