patch & patch.object Decorators

patch & patch.object Decorators

patch() temporarily replaces objects during tests, restoring them when the test finishes. It can be used as a decorator, context manager, or direct call.

patch as a Decorator

import unittest
from unittest.mock import patch

class TestUserService(unittest.TestCase):
    @patch('myapp.services.email.send_email')
    def test_welcome_email(self, mock_send_email):
        mock_send_email.return_value = True

        user = create_user("alice@example.com")

        mock_send_email.assert_called_once_with(
            to="alice@example.com",
            subject="Welcome!",
            body="Hello Alice"
        )

patch as a Context Manager

class TestConfig(unittest.TestCase):
    def test_production_mode(self):
        with patch('myapp.config.DEBUG', False):
            config = get_config()
            self.assertFalse(config.debug)

    def test_api_endpoint(self):
        with patch('myapp.services.api.BASE_URL', 'http://test-server'):
            response = call_api('/users')
            self.assertIsNotNone(response)

patch.object

class TestPayment(unittest.TestCase):
    def test_charge_card(self):
        gateway = PaymentGateway()

        with patch.object(gateway, 'charge', return_value=True):
            processor = PaymentProcessor(gateway)
            result = processor.charge_card(amount=100)

            self.assertTrue(result)
            gateway.charge.assert_called_once_with(amount=100)

Patch Target Paths

# patch the object WHERE IT IS USED, not where it's defined
# Correct: patches the name in myapp.services module
@patch('myapp.services.email.send_email')
def test_send(mock_send):
    pass

# Common pattern: mock at the import location
# If myapp.views imports email:
# from myapp.services.email import send_email
# Then patch 'myapp.views.send_email'

Key Takeaway

patch() and patch.object() replace objects during tests. Always patch where the object is used, not where it's defined. Use decorators for simple cases, context managers for multiple patches.