When Python imports a file, it runs every line in it. That's a problem for a file that also does things when you run it: importing it would print, ask for input or start a game.

Every module has a variable called __name__. When you run a file directly, Python sets it to "__main__". When the file is imported, it holds the module's name instead. So this pattern means "only do this when I'm the program being run":

def average(numbers):
    return sum(numbers) / len(numbers)

def main():
    marks = [70, 85, 64]
    print("Average:", average(marks))

if __name__ == "__main__":
    main()

Run it and it prints the average. Import it from another file and you just get the average function, ready to use. Putting the program's steps in a function called main keeps the top level of the file to definitions, which also makes every function easy to test on its own.

code = '''
def double(n):
    return n * 2

print("double.py sees __name__ =", __name__)
if __name__ == "__main__":
    print("double.py is running as a program")
'''
with open("double.py", "w") as f:
    f.write(code)

import double
print(double.double(21))
print("This program sees __name__ =", __name__)