Package calling functions from importing program

Question

Can a package call a function inside of the file that is importing it?

For example:

Code of file1.py

import file2

def onstart():
    print('Hello World!')

Code of file2.py

# ... something
onstart()

Then if I run file1, I want the output to be

Hello World!

I tried just calling it in file2.py, but that gave an error

NameError: name 'onstart' is not defined

Answer 1

I found a way. If I import __main__ in file2 and make a class in file2, if I call that class in file1, I can run functions from file1.

In file1

import file2

test = file2.Test()

def onstart():
    print('Hello World!')

test.run()

In file2

import __main__

class Test:
    def run(self):
        if hasattr(__main__,'onstart') and __main__.onstart:
            __main__.onstart()

Output

Hello World!

I found this answer by searching in Ursina's source code.