pytest: Writing and running tests in Python

In short: pytest is the most popular test framework for Python: tests are simple functions with assert, fixtures supply test data, and error messages are informative.

In more detail: With pytest you write tests without classes and without boilerplate. It finds files test_*.py and functions test_* automatically. Installation: pip install pytest.

In Depth

A simple test

# calc.py
def add(a, b):
    return a + b
 
# test_calc.py
from calc import add
 
def test_add_positive_numbers():
    assert add(2, 3) == 5
 
def test_add_negative_numbers():
    assert add(-1, -1) == -2

Start with pytest or pytest -v (verbose), pytest -k add (filter by name), pytest -x (stop at first failure). On failure pytest shows the compared values precisely.

Parametrised tests and exceptions

import pytest
 
@pytest.mark.parametrize("a, b, expected", [(1, 1, 2), (0, 0, 0), (-1, 1, 0)])
def test_add(a, b, expected):
    assert add(a, b) == expected
 
def test_division_by_zero():
    with pytest.raises(ZeroDivisionError):
        1 / 0

One test, many data sets: saves code and covers edge cases.

Fixtures

@pytest.fixture
def user():
    return {"name": "Mia", "age": 17}
 
def test_name(user):
    assert user["name"] == "Mia"

Fixtures provide reusable test preparation (database, temporary files with tmp_path, test clients) and clean up afterwards with yield. Shared fixtures live in conftest.py. With monkeypatch and unittest.mock you replace external dependencies.

Practice

• Structure: tests/ folder, tests independent of each other, fast unit tests first.
• Measure coverage: pytest --cov=package (plugin pytest-cov).
• Markers (@pytest.mark.slow) separate slow tests.
• CI: run pytest in GitHub Actions on every push.
• A test should check one thing and have an expressive name. TDD (test first) helps with design. Alternatives: unittest (in the standard library), nose (deprecated), hypothesis for property-based testing.

See also: Unit test, Mocking and test doubles, Code coverage, Property-based testing