Python 3: How to add a function to a Python class of a library?

If you want to add a method to one class the I think proper way to do that is to create class B that inherits from class A and there you can add the method. Something like

import open_source_library

class B(open_source_library.A):
    def my_fn():
         .....

Problem

# a.py module
def print_message(msg):
    print(msg)

# b.py module
from a import print_message
def execute():
    print_message("Hello")

# c.py module which will be executed
import b
b.execute()

Answer

import a

def _new_print_message(message):
    print("NEW: " + message)

a.print_message = _new_print_message

import b
b.execute()

You have to first import a, then override the function and then import b so that it would use the a module that is already imported (and changed).