Python Mock Patching Location

Python Mock Patching Location

Category: Testing Time Saved: 30+ minutes debugging "mock not working" Battle-tested: Yes — ChessCoach, multiple Python projects

The Problem

You're writing a test that needs to mock get_client() from module_a. You patch module_a.get_client. The mock doesn't work — the real function still runs.

Why It Happens

Python's import system creates a reference to the function in the importing module. When module_b does from module_a import get_client, it creates module_b.get_client as a separate reference. Patching the original doesn't affect the copy.

The Rule

Patch where the function is USED, not where it's DEFINED

# module_a.py defines get_client()
# module_b.py imports and calls it

# ❌ WRONG — patches the definition, not the usage
@patch('module_a.get_client')
def test_something(mock_client):
    result = module_b.do_work()  # Still calls real get_client!

# ✅ CORRECT — patches where it's called
@patch('module_b.get_client')
def test_something(mock_client):
    result = module_b.do_work()  # Uses mock

Decision Table

Async Context Manager Trap

MagicMock doesn't support async with by default:

# ❌ FAILS — MagicMock can't handle async context manager
mock_lock = MagicMock()
async with mock_lock:  # TypeError

# ✅ WORKS — proper async mock
class MockAsyncLock:
    async def __aenter__(self):
        return self
    async def __aexit__(self, *args):
        pass

mock_lock = MockAsyncLock()

Verification

Test passes AND you've confirmed the mock was actually called:

@patch('module_b.get_client')
def test_something(mock_client):
    mock_client.return_value = fake_response
    result = module_b.do_work()
    
    mock_client.assert_called_once()  # Verify mock was hit

If assert_called_once() fails with "Expected call not found", you patched the wrong location.

Source: Promoted from AI-Memory global-knowledge.md (2026-04-27)