pytest

When to use this skill

Use this skill whenever the user wants to:

• Write and run Python unit tests with pytest
• Use fixtures for test setup, teardown, and dependency injection
• Parametrize tests to run with multiple input sets
• Mock dependencies with pytest-mock or unittest.mock
• Configure pytest via pytest.ini, pyproject.toml, or conftest.py
• Generate coverage reports with pytest-cov
• Run tests in parallel with pytest-xdist

How to use this skill

Workflow

1. Write test functions with test_ prefix or Test class
2. Use fixtures in conftest.py for shared setup and dependency injection
3. Run tests with pytest CLI, applying markers and filters as needed
4. Analyze results with coverage and reporting plugins

1. Basic Test

def test_addition():
    assert 1 + 1 == 2

def test_string_upper():
    assert "hello".upper() == "HELLO"

2. Fixtures

import pytest

@pytest.fixture
def sample_user():
    return {"name": "Alice", "email": "[email protected]"}

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

@pytest.fixture(autouse=True)
def reset_database(db):
    db.clear()
    yield
    db.clear()

3. Parametrize

import pytest

@pytest.mark.parametrize("input,expected", [
    ("hello", 5),
    ("", 0),
    ("pytest", 6),
])
def test_string_length(input, expected):
    assert len(input) == expected

4. Mocking

def test_api_call(mocker):
    mock_get = mocker.patch("requests.get")
    mock_get.return_value.json.return_value = {"status": "ok"}

    result = fetch_status()
    assert result == "ok"
    mock_get.assert_called_once()

5. Running Tests

# Run all tests
pytest

# Verbose with pattern filter
pytest -v -k "test_user"

# With coverage
pytest --cov=mypackage --cov-report=html

# Parallel execution
pytest -n auto

Best Practices

• Keep tests independent with no shared mutable state; use fixtures for setup and teardown
• Use clear, descriptive test names that explain the expected behavior
• Avoid depending on test execution order; use conftest.py for shared fixtures
• Run the full test suite in CI; use --cov for coverage and --maxfail=1 to fail fast
• Use markers (@pytest.mark.slow) to categorize tests and run subsets

Resources

• Official documentation: https://docs.pytest.org/
• pytest-cov: https://pytest-cov.readthedocs.io/
• pytest-xdist: https://github.com/pytest-dev/pytest-xdist

Keywords

pytest, Python testing, fixtures, parametrize, mocking, conftest, coverage, pytest-cov, pytest-xdist, unit testing, markers