python-unit-tests

Overview

Per docs.pytest.org/en/stable (opens in new window):

pytest is the de facto Python test framework. Unlike stdlib unittest, it uses function-style tests (no TestCase), fixture-based dependency injection, parametrize for data-driven tests, and plain-assert rewriting for diff-rich failures.

Lifecycle scope: configure / run / fixtures / mocking / coverage / CI. Test code hygiene (assertions, AAA, mocking anti-patterns) is in test-code-conventions (qa-test-review plugin).

Choosing a framework

1. pytest for new code - the modern default; migration from unittest is mostly mechanical because pytest runs TestCase classes natively.
2. Match an existing unittest convention when maintaining a legacy suite or constrained to stdlib-only (no pip install) → references/unittest.md. unittest.mock is the canonical mocking library in either style.
3. doctest only for documentation-as-tests - executable examples in docstrings, not regression coverage → references/doctest.md.
4. Async code → the standalone pytest-asyncio-patterns skill (loop scoping, modes, AsyncMock).

Step 1 - Install

pip install pytest
# Common plugins:
pip install pytest-cov pytest-asyncio pytest-mock pytest-xdist

Step 2 - First test

# test_sum.py
def sum(a, b):
    return a + b

def test_adds_1_and_2():
    assert sum(1, 2) == 3

pytest

pytest auto-discovers via test_*.py / *_test.py filenames and test_* / Test* function/class names (pt-docs (opens in new window)).

Step 3 - Configuration

pytest.ini (or pyproject.toml [tool.pytest.ini_options] / setup.cfg [tool:pytest]):

# pyproject.toml
[tool.pytest.ini_options]
testpaths = ["tests"]
python_files = ["test_*.py", "*_test.py"]
python_classes = ["Test*"]
python_functions = ["test_*"]
addopts = "-ra --strict-markers --strict-config"
markers = [
    "slow: marks tests as slow (deselect with -m 'not slow')",
    "integration: marks tests requiring DB/external resources",
]

--strict-markers rejects undeclared marker names - catches typos like @pytest.mark.skipp (silently skipped before).

Step 4 - Fixtures

import pytest

@pytest.fixture
def db_connection():
    conn = create_connection()
    yield conn
    conn.close()

@pytest.fixture(scope="session")
def app_config():
    return load_config()

@pytest.fixture(autouse=True)
def reset_state():
    yield
    cleanup_after_test()

def test_user_creation(db_connection, app_config):
    user = create_user(db_connection, app_config)
    assert user.id is not None

Fixture scopes: function (default), class, module, package, session. Choose the narrowest scope that doesn't waste setup time. conftest.py shares fixtures across test files in the same directory (and subdirectories). Fixtures are requested by naming them as test-function parameters - pytest "searches for fixtures that have the same names as those parameters" (docs.pytest.org/how-to/fixtures (opens in new window)).

Step 5 - Parametrize

@pytest.mark.parametrize("a,b,expected", [
    (1, 2, 3),
    (0, 0, 0),
    (-1, 1, 0),
    (100, 200, 300),
])
def test_sum_parametrized(a, b, expected):
    assert sum(a, b) == expected

Stacked @pytest.mark.parametrize decorators multiply into a cross-product (docs.pytest.org/how-to/parametrize (opens in new window)).

Step 6 - Markers + skip/xfail

@pytest.mark.skip(reason="Requires staging DB")
def test_skip_example(): ...

@pytest.mark.skipif(sys.version_info < (3, 11), reason="Python 3.11+ syntax")
def test_modern_syntax(): ...

@pytest.mark.xfail(reason="Known bug; tracked in JIRA-1234")
def test_known_failure():
    assert 1 == 2

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

Filter: pytest -m "not slow" skips slow-marked tests.

Step 7 - Mocking with pytest-mock

def test_with_mock(mocker):
    mock_api = mocker.patch('mymodule.api_client.fetch')
    mock_api.return_value = {'id': 1, 'name': 'Alice'}

    result = my_function()
    mock_api.assert_called_once_with('/users')
    assert result == {'id': 1, 'name': 'Alice'}

The mocker fixture from pytest-mock wraps unittest.mock.patch with auto-cleanup at test end. Patch target rule: patch where the name is used, not where it's defined - if mymodule.py does from api import fetch_user, patch mymodule.fetch_user, not api.fetch_user (see references/unittest.md for the full unittest.mock catalog).

Step 8 - Coverage with pytest-cov

pytest --cov=src --cov-report=term-missing --cov-report=html --cov-report=xml \
       --cov-fail-under=80

--cov-fail-under=N fails the run if coverage drops below N%. Config-side equivalent in pyproject.toml:

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

[tool.coverage.report]
exclude_lines = [
    "pragma: no cover",
    "if TYPE_CHECKING:",
    "raise NotImplementedError",
]
fail_under = 80

branch = true enables branch (not just line) coverage; exclude_lines drops never-coverable lines from the denominator.

Step 9 - CI and parallel execution

- run: pip install -e .[dev]
- run: pytest --cov --cov-report=xml --cov-fail-under=80 --junitxml=junit.xml
- uses: codecov/codecov-action@v4
  with: { files: coverage.xml }

--junitxml=junit.xml emits a JUnit report (feeds junit-xml-analysis in qa-test-reporting); --cov-report=xml emits coverage.xml for the uploader. Parallel: pytest -n auto via pytest-xdist distributes tests across workers; pytest-cov merges per-worker data automatically.

Step 10 - Fast-feedback flags

pytest --lf            # only re-run last-failed tests
pytest --ff            # run last-failed first, then the rest
pytest -x              # stop on first failure
pytest -k "name_pat"   # only tests matching name pattern
pytest -s              # don't capture stdout (see print() output)

Authoring conventions

When authoring a new unit test in an existing project:

1. Detect the framework, never assume. Check pyproject.toml ([tool.pytest.ini_options] or pytest in dev-deps), setup.cfg ([tool:pytest]), tox.ini ([pytest]); then grep existing tests - unittest.TestCase subclasses → unittest; otherwise default to pytest. Doctest is opt-in per-module, only when the spec explicitly asks for in-docstring examples. Conflicting signals → stop and ask.
2. Match the layout. Existing tests/ dir → tests/test_<module>.py; co-located test_<module>.py → match it. For doctest, patch the source module's docstring instead of creating a file.
3. One spec → one new test; never modify existing test methods and never fabricate attributes/methods the target module does not expose.
4. Assert the spec's concrete outcome - no assert True / self.assertTrue(True) smoke asserts. Plain assert in pytest functions; self.assertEqual in TestCase classes (diff-aware).
5. Use present data peers only: mimesis in dev-deps → locale-aware fixtures via synthetic-data-toolkit (qa-test-data); never install packages as a side effect. 3+ interacting inputs → generate the case set with pairwise-test-case-generator (qa-test-data), then map through @pytest.mark.parametrize.
6. Refuse universally-quantified specs ("holds for all valid inputs") - that is property-based-test scope (qa-property-based plugin).
7. Beware mixed lifecycles: pytest's setup_method runs alongside (not instead of) setUp on TestCase subclasses - pick one mechanism per class. Avoid mutable default arguments in test helpers (shared across calls; leaks state).

Anti-patterns

Limitations

• Plugin ecosystem is large; conflicting plugins can cause subtle issues.
• Fixture-scope reasoning has a learning curve.
• assert rewriting requires pytest's importer; running tests as scripts bypasses it.
• Built-in async support is limited - use pytest-asyncio (see pytest-asyncio-patterns).

References

• pt-docs (opens in new window) - official pytest documentation
• docs.pytest.org/en/stable/how-to/fixtures.html - fixtures
• docs.pytest.org/en/stable/how-to/parametrize.html - parametrize
• pypi.org/project/pytest-mock - pytest-mock plugin
• pypi.org/project/pytest-xdist - parallel execution
• references/unittest.md - stdlib unittest + unittest.mock
• references/doctest.md - stdlib doctest
• pytest-asyncio-patterns - async test patterns (standalone sibling)
• test-code-conventions (qa-test-review) - test code hygiene