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.
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Comprehensive guide to implementing robust testing strategies in Python using pytest, fixtures, mocking, parameterization, and test-driven development practices.
# test_example.py
def add(a, b):
return a + b
def test_add():
"""Basic test example."""
result = add(2, 3)
assert result == 5
def test_add_negative():
"""Test with negative numbers."""
assert add(-1, 1) == 0
# Run with: pytest test_example.py
Detailed pattern documentation lives in references/details.md. Read that file when the navigation tier above is insufficient.
# tests/
# __init__.py
# conftest.py # Shared fixtures
# test_unit/ # Unit tests
# test_models.py
# test_utils.py
# test_integration/ # Integration tests
# test_api.py
# test_database.py
# test_e2e/ # End-to-end tests
# test_workflows.py
A common pattern: test_<unit>_<scenario>_<expected_outcome>. Adapt to your team's preferences.
# Pattern: test_<unit>_<scenario>_<expected>
def test_create_user_with_valid_data_returns_user():
...
def test_create_user_with_duplicate_email_raises_conflict():
...
def test_get_user_with_unknown_id_returns_none():
...
# Good test names - clear and descriptive
def test_user_creation_with_valid_data():
"""Clear name describes what is being tested."""
pass
def test_login_fails_with_invalid_password():
"""Name describes expected behavior."""
pass
def test_api_returns_404_for_missing_resource():
"""Specific about inputs and expected outcomes."""
pass
# Bad test names - avoid these
def test_1(): # Not descriptive
pass
def test_user(): # Too vague
pass
def test_function(): # Doesn't explain what's tested
pass
Verify that retry logic works correctly using mock side effects.
from unittest.mock import Mock
def test_retries_on_transient_error():
"""Test that service retries on transient failures."""
client = Mock()
# Fail twice, then succeed
client.request.side_effect = [
ConnectionError("Failed"),
ConnectionError("Failed"),
{"status": "ok"},
]
service = ServiceWithRetry(client, max_retries=3)
result = service.fetch()
assert result == {"status": "ok"}
assert client.request.call_count == 3
def test_gives_up_after_max_retries():
"""Test that service stops retrying after max attempts."""
client = Mock()
client.request.side_effect = ConnectionError("Failed")
service = ServiceWithRetry(client, max_retries=3)
with pytest.raises(ConnectionError):
service.fetch()
assert client.request.call_count == 3
def test_does_not_retry_on_permanent_error():
"""Test that permanent errors are not retried."""
client = Mock()
client.request.side_effect = ValueError("Invalid input")
service = ServiceWithRetry(client, max_retries=3)
with pytest.raises(ValueError):
service.fetch()
# Only called once - no retry for ValueError
assert client.request.call_count == 1
Use freezegun to control time in tests for predictable time-dependent behavior.
from freezegun import freeze_time
from datetime import datetime, timedelta
@freeze_time("2026-01-15 10:00:00")
def test_token_expiry():
"""Test token expires at correct time."""
token = create_token(expires_in_seconds=3600)
assert token.expires_at == datetime(2026, 1, 15, 11, 0, 0)
@freeze_time("2026-01-15 10:00:00")
def test_is_expired_returns_false_before_expiry():
"""Test token is not expired when within validity period."""
token = create_token(expires_in_seconds=3600)
assert not token.is_expired()
@freeze_time("2026-01-15 12:00:00")
def test_is_expired_returns_true_after_expiry():
"""Test token is expired after validity period."""
token = Token(expires_at=datetime(2026, 1, 15, 11, 30, 0))
assert token.is_expired()
def test_with_time_travel():
"""Test behavior across time using freeze_time context."""
with freeze_time("2026-01-01") as frozen_time:
item = create_item()
assert item.created_at == datetime(2026, 1, 1)
# Move forward in time
frozen_time.move_to("2026-01-15")
assert item.age_days == 14
# test_markers.py
import pytest
@pytest.mark.slow
def test_slow_operation():
"""Mark slow tests."""
import time
time.sleep(2)
@pytest.mark.integration
def test_database_integration():
"""Mark integration tests."""
pass
@pytest.mark.skip(reason="Feature not implemented yet")
def test_future_feature():
"""Skip tests temporarily."""
pass
@pytest.mark.skipif(os.name == "nt", reason="Unix only test")
def test_unix_specific():
"""Conditional skip."""
pass
@pytest.mark.xfail(reason="Known bug #123")
def test_known_bug():
"""Mark expected failures."""
assert False
# Run with:
# pytest -m slow # Run only slow tests
# pytest -m "not slow" # Skip slow tests
# pytest -m integration # Run integration tests
# Install coverage
pip install pytest-cov
# Run tests with coverage
pytest --cov=myapp tests/
# Generate HTML report
pytest --cov=myapp --cov-report=html tests/
# Fail if coverage below threshold
pytest --cov=myapp --cov-fail-under=80 tests/
# Show missing lines
pytest --cov=myapp --cov-report=term-missing tests/
For advanced patterns (async testing, monkeypatching, property-based testing, database testing, CI/CD integration, and configuration), see references/advanced-patterns.md
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
CLI to manage emails via IMAP/SMTP. Use `himalaya` to list, read, write, reply, forward, search, and organize emails from the terminal. Supports multiple accounts and message composition with MML (MIME Meta Language).
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
CLI to manage emails via IMAP/SMTP. Use `himalaya` to list, read, write, reply, forward, search, and organize emails from the terminal. Supports multiple accounts and message composition with MML (MIME Meta Language).