How do you mock dependencies without making tests brittle?

unittest.mock replaces an attribute for the duration of a test. The most common mistake is patching the defining module instead of the module under test, which leaves the code still bound to the real object.

from unittest.mock import patch

def test_charge_retries(monkeypatch):
    calls = []

    class FakeGateway:
        def charge(self, amount):
            calls.append(amount)
            return {"status": "ok"}

    service = PaymentService(gateway=FakeGateway())
    result = service.charge(500)

    assert result["status"] == "ok"
    assert calls == [500]

Mocking rules

• Patch where the object is used: patch the module under test, not the source module
• Use autospec=True so wrong argument signatures fail in the test
• Use side_effect to raise exceptions or return a sequence
• Use AsyncMock for async callables, never a plain MagicMock

Over-mocking couples tests to implementation. If a test asserts every internal call, a harmless refactor breaks it while a real bug can still pass. Prefer real value objects, small fakes and dependency injection, and reserve mocks for external systems such as networks, clocks and payment providers.