Agent skill

pytest-mastery

Python testing with pytest using uv package manager. Use when: (1) Running Python tests, (2) Writing test files or test functions, (3) Setting up fixtures, (4) Parametrizing tests, (5) Generating coverage reports, (6) Testing FastAPI applications, (7) Debugging test failures, (8) Configuring pytest options. Triggers: "run tests", "write tests", "test coverage", "pytest", "unit test", "integration test", "test FastAPI".

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SKILL.md

pytest Testing with uv

Quick Reference

bash
# Run all tests
uv run pytest

# Run with verbose output
uv run pytest -v

# Run specific file
uv run pytest tests/test_example.py

# Run specific test function
uv run pytest tests/test_example.py::test_function_name

# Run tests matching pattern
uv run pytest -k "pattern"

# Run with coverage
uv run pytest --cov=src --cov-report=html

Installation

bash
# Add pytest as dev dependency
uv add --dev pytest

# Add coverage support
uv add --dev pytest-cov

# Add async support (for FastAPI)
uv add --dev pytest-asyncio httpx

Test Discovery

pytest automatically discovers tests following these conventions:

  • Files: test_*.py or *_test.py
  • Functions: test_*
  • Classes: Test* (no __init__ method)
  • Methods: test_* inside Test* classes

Standard project structure:

project/
├── src/
│   └── myapp/
├── tests/
│   ├── __init__.py
│   ├── conftest.py      # Shared fixtures
│   ├── test_unit.py
│   └── integration/
│       └── test_api.py
└── pyproject.toml

Fixtures

Fixtures provide reusable test setup/teardown:

python
import pytest

@pytest.fixture
def sample_user():
    return {"id": 1, "name": "Test User"}

@pytest.fixture
def db_connection():
    conn = create_connection()
    yield conn  # Test runs here
    conn.close()  # Teardown

def test_user_name(sample_user):
    assert sample_user["name"] == "Test User"

Fixture Scopes

python
@pytest.fixture(scope="function")  # Default: new instance per test
@pytest.fixture(scope="class")     # Once per test class
@pytest.fixture(scope="module")    # Once per module
@pytest.fixture(scope="session")   # Once per test session

Shared Fixtures (conftest.py)

Place in tests/conftest.py for automatic availability:

python
# tests/conftest.py
import pytest

@pytest.fixture
def api_client():
    return TestClient(app)

Parametrization

Run same test with multiple inputs:

python
import pytest

@pytest.mark.parametrize("input,expected", [
    (1, 2),
    (2, 4),
    (3, 6),
])
def test_double(input, expected):
    assert input * 2 == expected

@pytest.mark.parametrize("value", [None, "", [], {}])
def test_falsy_values(value):
    assert not value

Common Options

Option Description
-v Verbose output
-vv More verbose
-q Quiet mode
-x Stop on first failure
--lf Run last failed tests only
--ff Run failures first
-k "expr" Filter by name expression
-m "mark" Run marked tests only
--tb=short Shorter traceback
--tb=no No traceback
-s Show print statements
--durations=10 Show 10 slowest tests
-n auto Parallel execution (pytest-xdist)

Coverage Reports

bash
# Terminal report
uv run pytest --cov=src

# HTML report (creates htmlcov/)
uv run pytest --cov=src --cov-report=html

# With minimum threshold (fails if below)
uv run pytest --cov=src --cov-fail-under=80

# Multiple report formats
uv run pytest --cov=src --cov-report=term --cov-report=xml

Markers

python
import pytest

@pytest.mark.slow
def test_slow_operation():
    ...

@pytest.mark.skip(reason="Not implemented")
def test_future_feature():
    ...

@pytest.mark.skipif(condition, reason="...")
def test_conditional():
    ...

@pytest.mark.xfail(reason="Known bug")
def test_known_failure():
    ...

Run by marker:

bash
uv run pytest -m "not slow"
uv run pytest -m "integration"

pyproject.toml Configuration

toml
[tool.pytest.ini_options]
testpaths = ["tests"]
python_files = ["test_*.py"]
python_functions = ["test_*"]
addopts = "-v --tb=short"
markers = [
    "slow: marks tests as slow",
    "integration: integration tests",
]

[tool.coverage.run]
source = ["src"]
omit = ["tests/*", "*/__init__.py"]

[tool.coverage.report]
exclude_lines = [
    "pragma: no cover",
    "if TYPE_CHECKING:",
]

FastAPI Testing

See references/fastapi-testing.md for comprehensive FastAPI testing patterns including:

  • TestClient setup
  • Async testing with httpx
  • Database fixture patterns
  • Dependency overrides
  • Authentication testing

Debugging Failed Tests

bash
# Run with full traceback
uv run pytest --tb=long

# Drop into debugger on failure
uv run pytest --pdb

# Show local variables in traceback
uv run pytest -l

# Run only previously failed
uv run pytest --lf

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