Mock Objects with unittest.mock

Mock Objects with unittest.mock

Mock objects replace real objects in your tests, allowing you to control their behavior and verify interactions. The unittest.mock module provides powerful mocking capabilities without external dependencies.

Creating a Mock

import unittest
from unittest.mock import Mock

class TestWithMock(unittest.TestCase):
    def test_mock_method(self):
        mock_service = Mock()
        mock_service.get_user.return_value = {"name": "Alice"}

        result = mock_service.get_user(1)
        self.assertEqual(result["name"], "Alice")
        mock_service.get_user.assert_called_once_with(1)

    def test_mock_not_called(self):
        mock_service = Mock()
        # Don't call any methods
        mock_service.get_user.assert_not_called()

Mocking External Services

from unittest.mock import Mock, patch

class TestPayment(unittest.TestCase):
    def test_process_payment(self):
        payment_gateway = Mock()
        payment_gateway.charge.return_value = {"status": "success", "id": "txn_123"}

        processor = PaymentProcessor(payment_gateway)
        result = processor.process_payment(amount=50.00)

        self.assertTrue(result.success)
        payment_gateway.charge.assert_called_once_with(amount=50.00)

Mock Return Values

mock_obj = Mock()

# Single return value
mock_obj.method.return_value = 42

# Side effects (exceptions)
from unittest.mock import side_effect
mock_obj.method.side_effect = ConnectionError("Timeout")

# Side effects (multiple values)
mock_obj.method.side_effect = [1, 2, 3]

# Usage
print(mock_obj.method())  # 1
print(mock_obj.method())  # 2
print(mock_obj.method())  # 3

Key Takeaway

Mock objects let you replace dependencies with controllable fakes. Use return_value to set what methods return, and side_effect for exceptions or dynamic behavior.