Pytest

Pytest is Python’s de-facto testing framework, built around plain assert statements, fixtures for setup/teardown, and a plugin ecosystem (coverage, freezing time, mocking).

Monkeypatching a Method

The monkeypatch fixture replaces an attribute for the duration of a test and restores it automatically afterward. Wrapping it in an autouse=True fixture applies the patch to every test in the module without an explicit request.

import pytest
from your_module import Component
 
@pytest.fixture(autouse=True)
def patch_set_name(monkeypatch):
    def mock_set_name(self, name):
        pass  # customize per test
 
    monkeypatch.setattr(Component, "set_name", mock_set_name)

Adding Directories to $PYTHONPATH

Rather than mutating sys.path in test code, declare extra import paths in config so every test run picks them up consistently. In pytest.ini:

[pytest]
pythonpath = ADDITIONAL_DIRECTORY1 ADDITIONAL_DIRECTORY2

Or in pyproject.toml:

[tool.pytest.ini_options]
pythonpath = ["ADDITIONAL_DIRECTORY1", "ADDITIONAL_DIRECTORY2"]

Fixture Cleanup

A fixture that returns only sets up state. Swap return for yield and put teardown code after the yield — pytest runs everything past it once the test finishes, even if the test raises.

@pytest.fixture
def resource():
    r = acquire()
    yield r
    r.release()  # runs after the test, even on failure

Using a Fixture Inside pytest.mark.parametrize

parametrize values are resolved before fixtures run, so you can’t pass a fixture directly as a parameter. The workaround is the built-in request fixture, which exposes getfixturevalue(fixture_name: str) to resolve a fixture by name at test time.

@pytest.mark.parametrize("fixture_name", ["fixture_a", "fixture_b"])
def test_something(request, fixture_name):
    value = request.getfixturevalue(fixture_name)

Coverage

Install pytest-cov and run with --cov:

pytest --cov-report html --cov=.

Exclude folders (e.g. the test suite itself) via .coveragerc:

[run]
omit = tests/*

Cheatsheet

Monkeypatching

monkeypatch.setattr(Component, "set_name", mock_set_name)

Config

# pytest.ini
[pytest]
pythonpath = ADDITIONAL_DIRECTORY1 ADDITIONAL_DIRECTORY2

# pyproject.toml
[tool.pytest.ini_options]
pythonpath = ["ADDITIONAL_DIRECTORY1", "ADDITIONAL_DIRECTORY2"]

Fixtures

yield r  # instead of return, for teardown after
request.getfixturevalue(fixture_name)  # resolve a fixture by name

Coverage

pytest --cov-report html --cov=. # requires pytest-cov

# .coveragerc
[run]
omit = tests/*