lab_ai/.agents/skills/pytest/references/mocking.md

Mocking

Table of Contents

• Basic Mocking
• Patching
• Mock Objects
• Async Mocking
• Fixture-Based Mocking
• Common Patterns

Basic Mocking

MagicMock Basics

from unittest.mock import MagicMock

def test_with_mock():
    # Create a mock object
    mock_service = MagicMock()

    # Configure return value
    mock_service.get_data.return_value = {"id": 1, "name": "Test"}

    # Use mock
    result = mock_service.get_data()
    assert result["name"] == "Test"

    # Verify call
    mock_service.get_data.assert_called_once()

Return Value Configuration

from unittest.mock import MagicMock

mock = MagicMock()

# Simple return value
mock.method.return_value = 42

# Different returns on consecutive calls
mock.method.side_effect = [1, 2, 3]
assert mock.method() == 1
assert mock.method() == 2
assert mock.method() == 3

# Raise exception
mock.method.side_effect = ValueError("Error!")

# Dynamic return value
mock.method.side_effect = lambda x: x * 2
assert mock.method(5) == 10

Patching

patch Decorator

from unittest.mock import patch

# Patch a function
@patch("mymodule.external_api")
def test_with_patched_api(mock_api):
    mock_api.return_value = {"status": "ok"}
    result = mymodule.call_api()
    assert result["status"] == "ok"

# Patch multiple
@patch("mymodule.function_a")
@patch("mymodule.function_b")
def test_multiple_patches(mock_b, mock_a):
    # Note: decorators apply bottom-up
    mock_a.return_value = "a"
    mock_b.return_value = "b"

patch Context Manager

from unittest.mock import patch

def test_with_context_manager():
    with patch("mymodule.external_api") as mock_api:
        mock_api.return_value = {"data": "mocked"}
        result = mymodule.process()
        assert result == {"data": "mocked"}
    # Original function restored after with block

patch.object

from unittest.mock import patch

class MyService:
    def fetch_data(self):
        return "real data"

def test_patch_instance_method():
    service = MyService()

    with patch.object(service, "fetch_data", return_value="mocked"):
        assert service.fetch_data() == "mocked"

Patching Where Used

# mymodule.py
from requests import get  # 'get' is imported here

def fetch_url(url):
    return get(url).text

# test_mymodule.py
# Patch where it's USED, not where it's DEFINED
@patch("mymodule.get")  # NOT "requests.get"
def test_fetch_url(mock_get):
    mock_get.return_value.text = "mocked response"
    result = fetch_url("http://example.com")
    assert result == "mocked response"

Mock Objects

Spec and Autospec

from unittest.mock import MagicMock, create_autospec

class UserService:
    def get_user(self, user_id: int) -> dict:
        pass

    def create_user(self, data: dict) -> dict:
        pass

# Basic mock (allows any attribute)
mock = MagicMock()
mock.nonexistent_method()  # Works, but shouldn't

# Spec mock (restricts to real attributes)
mock = MagicMock(spec=UserService)
# mock.nonexistent_method()  # Raises AttributeError

# Autospec (also checks signatures)
mock = create_autospec(UserService)
# mock.get_user()  # Raises TypeError (missing user_id)
mock.get_user(123)  # Works

Assertion Methods

from unittest.mock import MagicMock, call

mock = MagicMock()
mock.method(1, 2, key="value")
mock.method(3, 4)

# Verify calls
mock.method.assert_called()
mock.method.assert_called_once()  # Fails - called twice
mock.method.assert_called_with(3, 4)
mock.method.assert_any_call(1, 2, key="value")

# Check call count
assert mock.method.call_count == 2

# Check all calls
assert mock.method.call_args_list == [
    call(1, 2, key="value"),
    call(3, 4),
]

# Reset mock
mock.reset_mock()
assert mock.method.call_count == 0

Async Mocking

AsyncMock

from unittest.mock import AsyncMock, patch
import pytest

@pytest.mark.asyncio
async def test_async_mock():
    mock = AsyncMock(return_value={"data": "mocked"})
    result = await mock()
    assert result["data"] == "mocked"

@pytest.mark.asyncio
@patch("mymodule.async_fetch", new_callable=AsyncMock)
async def test_patched_async(mock_fetch):
    mock_fetch.return_value = {"status": "ok"}
    result = await mymodule.process()
    assert result["status"] == "ok"

Async Side Effects

from unittest.mock import AsyncMock

@pytest.mark.asyncio
async def test_async_side_effect():
    mock = AsyncMock()

    # Return different values
    mock.side_effect = [1, 2, 3]
    assert await mock() == 1
    assert await mock() == 2

    # Async function as side effect
    async def async_side_effect(x):
        return x * 2

    mock.side_effect = async_side_effect
    assert await mock(5) == 10

Fixture-Based Mocking

Mock Fixtures

import pytest
from unittest.mock import MagicMock, patch

@pytest.fixture
def mock_database():
    """Fixture providing mock database."""
    mock_db = MagicMock()
    mock_db.query.return_value = [{"id": 1}, {"id": 2}]
    return mock_db

def test_with_mock_fixture(mock_database):
    result = mock_database.query("SELECT * FROM users")
    assert len(result) == 2

@pytest.fixture
def mock_external_api():
    """Fixture with patching."""
    with patch("mymodule.external_api") as mock:
        mock.return_value = {"status": "ok"}
        yield mock

Monkeypatch Fixture

def test_with_monkeypatch(monkeypatch):
    # Patch function
    monkeypatch.setattr("mymodule.get_config", lambda: {"debug": True})

    # Patch environment variable
    monkeypatch.setenv("API_KEY", "test-key")

    # Patch dictionary item
    monkeypatch.setitem(mymodule.settings, "DEBUG", True)

    # Patch attribute
    monkeypatch.setattr(mymodule.client, "timeout", 5)

Common Patterns

Mock HTTP Responses

from unittest.mock import patch, MagicMock

@patch("requests.get")
def test_http_request(mock_get):
    # Configure mock response
    mock_response = MagicMock()
    mock_response.status_code = 200
    mock_response.json.return_value = {"data": "test"}
    mock_get.return_value = mock_response

    result = fetch_data("http://api.example.com")

    assert result == {"data": "test"}
    mock_get.assert_called_once_with("http://api.example.com")

Mock File Operations

from unittest.mock import mock_open, patch

def test_read_file():
    mock_file_content = "Hello, World!"

    with patch("builtins.open", mock_open(read_data=mock_file_content)):
        result = read_file("test.txt")
        assert result == "Hello, World!"

def test_write_file():
    m = mock_open()

    with patch("builtins.open", m):
        write_file("test.txt", "content")

    m().write.assert_called_once_with("content")

Mock Datetime

from unittest.mock import patch
from datetime import datetime

@patch("mymodule.datetime")
def test_with_frozen_time(mock_datetime):
    mock_datetime.now.return_value = datetime(2024, 1, 15, 12, 0, 0)
    mock_datetime.side_effect = lambda *args, **kwargs: datetime(*args, **kwargs)

    result = mymodule.get_timestamp()
    assert result == "2024-01-15 12:00:00"

Mock Class Instance

from unittest.mock import patch, MagicMock

class EmailService:
    def send(self, to, subject, body):
        # Real implementation
        pass

@patch("mymodule.EmailService")
def test_email_sent(MockEmailService):
    # Configure mock instance
    mock_instance = MagicMock()
    MockEmailService.return_value = mock_instance

    # Call function that uses EmailService
    send_notification("user@example.com", "Hello!")

    # Verify email was sent
    mock_instance.send.assert_called_once_with(
        "user@example.com",
        "Notification",
        "Hello!",
    )