Calling a function in a separate file in Python [closed]

I've read the following posts:

Importing Module or From Module Import

From file.py import *

And I was just wondering how to know when to break-up my code into multiple files versus putting many functions in one file? My specific problem here is that I have a function with 100 lines that I want to call in the for-loop of another function. Also, when are scripts executed? When you import them, or when you call them?

Note: The answers below have fully solved the problem. Thank you!

Solution 1:

Assuming that the function useful_function is in a file foreign_code.py in the same directory as your program file, just put

from foreign_code import useful_function

at the top of your program.

Solution 2:

Depending on the nature of the other file, importing it may be a good solution.

from otherfile import big_function

for something something:
    big_function()