Why does two main functions in a single Python script work?

Question

I was trying to do silly things in Python and tried the silliest of the things (see below) to see how Python reacts. To my surprise it executed perfectly. But I do not understand why.

How does Python know which foo to execute? Why does it not execute the same foo twice?

def main():
    foo()

def foo():
    print('this is foo 1.')

if __name__ == '__main__':
    main()

def main():
    foo()

def foo():
    print('this is foo 2.')

if __name__ == '__main__':
    main()

Asked By: Gahlot

||

Source

Answer 1

Python executes statements from top to bottom as they appear in the input file.

So first it defines two functions main and foo and then calls main, leading to the output from foo, "this is foo 1".

Then it defines two other functions which happen to be also named main and foo, these names now refer to the new functions and the first two functions are no longer accessible by these names. You could also say that the new functions override the old functions or that they are redefined.

Then it calls main (which now refers to the new function) which leads to the output of the new function foo, "this is foo 2".

Also note that the function name main has no special meaning in Python.

Answered By: mkrieger1

Answer 2

When main is called the first time, foo is the function that prints "This is foo 1.". When main is called the second time, foo is the function that prints "This is foo 2." Therefore, the first time, "This is foo 1." is the output, and the second time, "This is foo 2." is the output.

Answered By: The Thonnu

Answer 3

Python is in interpretted language so it executes each line as it encounters it.

Which means if you ran this on the python shell or as a script your output would be determined by whatever is encountered first foo() – foo1, and then the newer value of the function foo() – foo2

Output:

this is foo 1.

this is foo 2.

Answered By: newton

Answer 4

The interpreter does these things, in the order they appear in the file:

Defines main as your first definition of that function.
Defines foo as your first definition of that function.
Because __name__ is __main__: executes the function (currently defined as main) which calls the function currently defined as foo.
Redefines main as your second definition of that function. (Albeit that’s no different from the first definition.)
Redefines foo as your second definition of that function.
Because __name__ is __main__: executes the function (currently defined as main) which calls the function currently defined as foo, which by now is your second foo.

You might be assuming that the interpreter somehow does a "first pass" and processes all function definitions, and only then executes the code under if __name__ == '__main__'. But that’s not how it works. The steps are executed in order, like they would be if you ran:

x = 5
y = 7
print(x+y)
x = 6
y = 10
print(x+y)

For similar reasons, this code would not work:

if __name__ == '__main__':
   foo()

def foo():
   print(2)

because foo is undefined at the point it is referenced.

Answered By: slothrop

Why does two main functions in a single Python script work?

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