Teenage Mutant Ninja Modules

README.md

The Scenery

There are 3 modules: m1.py, m2.py and main.py. We can only change the code of main.py and inside it we want to call a function from m1: m1.m1(). However, hidden deep inside the spaghetti codebase m1 is calling m2.m2() on the top level, so it gets executed every time we import m1. And we don't want that

The Problem

How can we import m1 without m2.m2() getting executed and without altering any code inside m1.py and m2.py?

The Solution

We mutate the m2 module inside main before m1 is imported.

m1.py

import m2

m2.m2()  # well.. this shouldn't be happening


def m1():
    print('m1')

m2.py

def m2():
    print('m2')

main.py

# We don't want m1 to call m2.m2(), so we mutate it
import m2
m2.m2 = lambda *args, **kwargs: None

import m1

m1.m1()