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 Standard Testing Framework: The most widely adopted, powerful testing framework for Python web backends, data science, and ML.
• Clean Fixture Dependency Injection: Managing test setup, database connections, and teardown cleanly with @pytest.fixture.
• Parameterized Testing: Running test functions across multi-dimensional input sets with @pytest.mark.parametrize.
• Rich Plugin Ecosystem: Extending capabilities via pytest-asyncio, pytest-cov, pytest-xdist, and pytest-mock.

Quick Start

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

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

Running it:

```bash
pytest

Core Concepts

Dependency Injection with Fixtures

Fixtures provide modular, reusable dependencies with explicit lifecycle scopes:

# conftest.py
import pytest
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker

@pytest.fixture(scope="session")
def db_engine():
    engine = create_engine("sqlite:///:memory:")
    yield engine
    engine.dispose()

@pytest.fixture
def db_session(db_engine):
    Session = sessionmaker(bind=db_engine)
    session = Session()
    yield session
    session.rollback()
    session.close()

Parameterized Tests (@pytest.mark.parametrize)

Tests multiple input/output scenarios cleanly without loop boilerplate:

import pytest

@pytest.mark.parametrize("email,expected_valid", [
    ("user@example.com", True),
    ("invalid-email", False),
    ("@missinguser.com", False),
    ("user.name+tag@sub.domain.co", True),
])
def test_email_validation(email, expected_valid):
    assert validate_email(email) == expected_valid

Async Testing with pytest-asyncio

Tests async coroutines seamlessly:

import pytest
import httpx

@pytest.mark.asyncio
async def test_async_fetch():
    async with httpx.AsyncClient() as client:
        response = await client.get("https://httpbin.org/get")
        assert response.status_code == 200

Common Patterns

Scoped Fixtures with Teardown / Cleanup

Problem: Database or mock setup code repeated across dozens of test functions creates duplication and leftover state.

Solution: Use @pytest.fixture with the yield statement:

import pytest

@pytest.fixture(scope="function")
def db_session():
    # Setup test database session
    session = create_test_session()
    yield session
    # Cleanup and rollback after test completes
    session.rollback()
    session.close()

def test_user_creation(db_session):
    user = db_session.create_user("alice@example.com")
    assert user.id is not None

Best Practices

Do:

• Use Plain assert Statements: Pytest provides detailed assertion introspection without requiring special assertion methods.
• Use conftest.py for Shared Fixtures: Place global fixtures and configuration hooks in conftest.py files.
• Run Parallel Tests with pytest-xdist: Accelerate test runs across all CPU cores with pytest -n auto.
• Mark Tests by Category: Use custom marks (@pytest.mark.slow, @pytest.mark.integration) and filter runs with -m "not slow".

Don't:

• Use assert with parentheses: Writing assert(a == b, "msg") evaluates a non-empty tuple which is always truthy.
• Mutate shared session-scoped fixtures: Keep session fixtures read-only; use function-scoped fixtures for test-isolated mutations.
• Create deep nested class hierarchies: Pytest favors simple standalone test functions over unittest-style classes.

Troubleshooting

Error	Cause	Solution
ModuleNotFoundError: No module named 'src'	Current directory not in sys.path when running pytest.	Run pytest as a module: python -m pytest, or configure pythonpath = ["src"] in pyproject.toml.
FixtureNotFound: fixture '...' not found	Typo in fixture argument name or fixture defined outside conftest.py.	Move shared fixtures to root conftest.py file.
Failed: DID NOT RAISE <class '...'>	Code did not raise the exception expected by pytest.raises.	Inspect inputs to verify failure condition is actually triggered.

References

• Pytest Documentation