Pytest

Pytest

Pytest is the dominant testing framework for Python. It is loved for its no-boilerplate syntax (assert x == y), powerful fixture system, and rich plugin ecosystem.

When to Use

• Python Projects: The standard for 99% of new Python projects.
• Complex Setup: Use Fixtures to handle database connections, API clients, or mock data.
• Parametrization: Running the same test with different inputs.

Quick Start

# content of test_sample.py
def inc(x):
    return x + 1

def test_answer():
    assert inc(3) == 5

Running it:

pytest

Core Concepts

Fixtures

Functions that run before tests to set up state. They are injected via argument name matching.

import pytest

@pytest.fixture
def database():
    db = connect_db()
    yield db
    db.close()

def test_insert(database):
    database.insert("user")
    assert database.count() == 1

Parametrization

Decorating a test to run multiple times.

@pytest.mark.parametrize("input,expected", [
    ("3+5", 8),
    ("2+4", 6),
])
def test_eval(input, expected):
    assert eval(input) == expected

Best Practices (2025)

Do:

• Use conftest.py: Place shared fixtures here. Pytest discovers them automatically.
• Use pytest-cov: For coverage reports (pytest --cov=src).
• Use Markers: Tag slow tests (@pytest.mark.slow) and exclude them during dev (pytest -m "not slow").

Don't:

• Don't use unittest.TestCase: Unless migrating legacy code. Functional tests are cleaner.
• Don't use global state: Fixtures should return fresh instances.

References

• Pytest Documentation