1---
2name: python-testing-patterns
3description: Implement comprehensive testing strategies with pytest, fixtures, mocking, and test-driven development. Use when writing Python tests, setting up test suites, or implementing testing best practices.
4---
5 
6# Python Testing Patterns
7 
8Comprehensive guide to implementing robust testing strategies in Python using pytest, fixtures, mocking, parameterization, and test-driven development practices.
9 
10## When to Use This Skill
11 
12- Writing unit tests for Python code
13- Setting up test suites and test infrastructure
14- Implementing test-driven development (TDD)
15- Creating integration tests for APIs and services
16- Mocking external dependencies and services
17- Testing async code and concurrent operations
18- Setting up continuous testing in CI/CD
19- Implementing property-based testing
20- Testing database operations
21- Debugging failing tests
22 
23## Core Concepts
24 
25### 1. Test Types
26 
27- **Unit Tests**: Test individual functions/classes in isolation
28- **Integration Tests**: Test interaction between components
29- **Functional Tests**: Test complete features end-to-end
30- **Performance Tests**: Measure speed and resource usage
31 
32### 2. Test Structure (AAA Pattern)
33 
34- **Arrange**: Set up test data and preconditions
35- **Act**: Execute the code under test
36- **Assert**: Verify the results
37 
38### 3. Test Coverage
39 
40- Measure what code is exercised by tests
41- Identify untested code paths
42- Aim for meaningful coverage, not just high percentages
43 
44### 4. Test Isolation
45 
46- Tests should be independent
47- No shared state between tests
48- Each test should clean up after itself
49 
50## Quick Start
51 
52```python
53# test_example.py
54def add(a, b):
55    return a + b
56 
57def test_add():
58    """Basic test example."""
59    result = add(2, 3)
60    assert result == 5
61 
62def test_add_negative():
63    """Test with negative numbers."""
64    assert add(-1, 1) == 0
65 
66# Run with: pytest test_example.py
67```
68 
69## Detailed patterns and worked examples
70 
71Detailed pattern documentation lives in `references/details.md`. Read that file when the navigation tier above is insufficient.
72 
73## Testing Best Practices
74 
75### Test Organization
76 
77```python
78# tests/
79#   __init__.py
80#   conftest.py           # Shared fixtures
81#   test_unit/            # Unit tests
82#     test_models.py
83#     test_utils.py
84#   test_integration/     # Integration tests
85#     test_api.py
86#     test_database.py
87#   test_e2e/            # End-to-end tests
88#     test_workflows.py
89```
90 
91### Test Naming Convention
92 
93A common pattern: `test_<unit>_<scenario>_<expected_outcome>`. Adapt to your team's preferences.
94 
95```python
96# Pattern: test_<unit>_<scenario>_<expected>
97def test_create_user_with_valid_data_returns_user():
98    ...
99 
100def test_create_user_with_duplicate_email_raises_conflict():
101    ...
102 
103def test_get_user_with_unknown_id_returns_none():
104    ...
105 
106# Good test names - clear and descriptive
107def test_user_creation_with_valid_data():
108    """Clear name describes what is being tested."""
109    pass
110 
111def test_login_fails_with_invalid_password():
112    """Name describes expected behavior."""
113    pass
114 
115def test_api_returns_404_for_missing_resource():
116    """Specific about inputs and expected outcomes."""
117    pass
118 
119# Bad test names - avoid these
120def test_1():  # Not descriptive
121    pass
122 
123def test_user():  # Too vague
124    pass
125 
126def test_function():  # Doesn't explain what's tested
127    pass
128```
129 
130### Testing Retry Behavior
131 
132Verify that retry logic works correctly using mock side effects.
133 
134```python
135from unittest.mock import Mock
136 
137def test_retries_on_transient_error():
138    """Test that service retries on transient failures."""
139    client = Mock()
140    # Fail twice, then succeed
141    client.request.side_effect = [
142        ConnectionError("Failed"),
143        ConnectionError("Failed"),
144        {"status": "ok"},
145    ]
146 
147    service = ServiceWithRetry(client, max_retries=3)
148    result = service.fetch()
149 
150    assert result == {"status": "ok"}
151    assert client.request.call_count == 3
152 
153def test_gives_up_after_max_retries():
154    """Test that service stops retrying after max attempts."""
155    client = Mock()
156    client.request.side_effect = ConnectionError("Failed")
157 
158    service = ServiceWithRetry(client, max_retries=3)
159 
160    with pytest.raises(ConnectionError):
161        service.fetch()
162 
163    assert client.request.call_count == 3
164 
165def test_does_not_retry_on_permanent_error():
166    """Test that permanent errors are not retried."""
167    client = Mock()
168    client.request.side_effect = ValueError("Invalid input")
169 
170    service = ServiceWithRetry(client, max_retries=3)
171 
172    with pytest.raises(ValueError):
173        service.fetch()
174 
175    # Only called once - no retry for ValueError
176    assert client.request.call_count == 1
177```
178 
179### Mocking Time with Freezegun
180 
181Use freezegun to control time in tests for predictable time-dependent behavior.
182 
183```python
184from freezegun import freeze_time
185from datetime import datetime, timedelta
186 
187@freeze_time("2026-01-15 10:00:00")
188def test_token_expiry():
189    """Test token expires at correct time."""
190    token = create_token(expires_in_seconds=3600)
191    assert token.expires_at == datetime(2026, 1, 15, 11, 0, 0)
192 
193@freeze_time("2026-01-15 10:00:00")
194def test_is_expired_returns_false_before_expiry():
195    """Test token is not expired when within validity period."""
196    token = create_token(expires_in_seconds=3600)
197    assert not token.is_expired()
198 
199@freeze_time("2026-01-15 12:00:00")
200def test_is_expired_returns_true_after_expiry():
201    """Test token is expired after validity period."""
202    token = Token(expires_at=datetime(2026, 1, 15, 11, 30, 0))
203    assert token.is_expired()
204 
205def test_with_time_travel():
206    """Test behavior across time using freeze_time context."""
207    with freeze_time("2026-01-01") as frozen_time:
208        item = create_item()
209        assert item.created_at == datetime(2026, 1, 1)
210 
211        # Move forward in time
212        frozen_time.move_to("2026-01-15")
213        assert item.age_days == 14
214```
215 
216### Test Markers
217 
218```python
219# test_markers.py
220import pytest
221 
222@pytest.mark.slow
223def test_slow_operation():
224    """Mark slow tests."""
225    import time
226    time.sleep(2)
227 
228 
229@pytest.mark.integration
230def test_database_integration():
231    """Mark integration tests."""
232    pass
233 
234 
235@pytest.mark.skip(reason="Feature not implemented yet")
236def test_future_feature():
237    """Skip tests temporarily."""
238    pass
239 
240 
241@pytest.mark.skipif(os.name == "nt", reason="Unix only test")
242def test_unix_specific():
243    """Conditional skip."""
244    pass
245 
246 
247@pytest.mark.xfail(reason="Known bug #123")
248def test_known_bug():
249    """Mark expected failures."""
250    assert False
251 
252 
253# Run with:
254# pytest -m slow          # Run only slow tests
255# pytest -m "not slow"    # Skip slow tests
256# pytest -m integration   # Run integration tests
257```
258 
259### Coverage Reporting
260 
261```bash
262# Install coverage
263pip install pytest-cov
264 
265# Run tests with coverage
266pytest --cov=myapp tests/
267 
268# Generate HTML report
269pytest --cov=myapp --cov-report=html tests/
270 
271# Fail if coverage below threshold
272pytest --cov=myapp --cov-fail-under=80 tests/
273 
274# Show missing lines
275pytest --cov=myapp --cov-report=term-missing tests/
276```
277 
278For advanced patterns (async testing, monkeypatching, property-based testing, database testing, CI/CD integration, and configuration), see [references/advanced-patterns.md](references/advanced-patterns.md)
279