Why use def main()? [duplicate]
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
importthat 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.
def mainare 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
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
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 sysdef main(argv): # My code here passif __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 hereif __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 :
In this case
foo, the code section does not get executed and does not print
- executed directly :
When it is executed directly,
__name__ is same as
__main__ and the code in that section is executed and prints
One of the use of this functionality to write various kind of unit tests within the same module.