Python Testing with pytest: Fixtures, Mocking and Coverage

Python Testing with pytest

pytest is the most popular Python testing framework. More concise than unittest, with excellent fixture support and parametrization.

def test_addition():
    assert 2 + 2 == 4

def test_raises():
    import pytest
    with pytest.raises(ValueError, match="invalid"):
        validate_email("not-an-email")

Fixtures

import pytest

@pytest.fixture
def sample_user():
    return {'id': 1, 'name': 'Rahul', 'email': 'r@e.com', 'active': True}

@pytest.fixture
def db():
    connection = create_test_db()
    yield connection
    connection.close()

def test_user_is_active(sample_user):
    assert sample_user['active'] is True

def test_user_in_db(db, sample_user):
    db.insert('users', sample_user)
    result = db.find('users', id=1)
    assert result['name'] == 'Rahul'

Parametrize and Mocks

import pytest
from unittest.mock import patch

@pytest.mark.parametrize("email,valid", [
    ("rahul@example.com", True),
    ("bad-email", False),
    ("@nodomain.com", False),
])
def test_email_validation(email, valid):
    assert validate_email(email) == valid

def test_send_email():
    with patch('myapp.services.send_email') as mock_send:
        mock_send.return_value = True
        result = UserService.register({'name': 'Rahul', 'email': 'r@e.com'})
        mock_send.assert_called_once_with('r@e.com', subject='Welcome!')

pytest -v                    # verbose
pytest -k "test_user"        # run matching tests
pytest --cov=myapp --cov-report=html  # coverage report