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,