Python Mock Pitfall: Patch Where It Is Used, Not Where It Is Defined

Originally published at recca0120.github.io from utils import sum , then patch('utils.sum') — and the mock never takes effect. Switch to patch('helloworld.sum') and it works. This is the most common Python mock mistake. Once you understand why, you won't hit it again. Reproducing the Problem utils.py : def sum ( a , b ): return a + b helloworld.py : from utils import sum def main (): return sum ( 1 , 2 ) Writing a test to mock sum : from unittest.mock import patch def test_main (): with patch (