Why use def main()? [duplicate]

Question

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?

Answer 1

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

Answer 2

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. :)