pytest - Professional Python Testing

Overview

pytest is the industry-standard Python testing framework, offering powerful features like fixtures, parametrization, markers, plugins, and seamless integration with FastAPI, Django, and Flask. It provides a simple, scalable approach to testing from unit tests to complex integration scenarios.

Key Features:

• Fixture system for dependency injection
• Parametrization for data-driven tests
• Rich assertion introspection (no need for self.assertEqual)
• Plugin ecosystem (pytest-cov, pytest-asyncio, pytest-mock, pytest-django)
• Async/await support
• Parallel test execution with pytest-xdist
• Test discovery and organization
• Detailed failure reporting

Installation:

# Basic pytest
pip install pytest

# With common plugins
pip install pytest pytest-cov pytest-asyncio pytest-mock

# For FastAPI testing
pip install pytest httpx pytest-asyncio

# For Django testing
pip install pytest pytest-django

# For async databases
pip install pytest-asyncio aiosqlite

Basic Testing Patterns

1. Simple Test Functions

# test_math.py
def add(a, b):
    return a + b

def test_add():
    assert add(2, 3) == 5
    assert add(-1, 1) == 0
    assert add(0, 0) == 0

def test_add_negative():
    assert add(-2, -3) == -5

Run tests:

# Discover and run all tests
pytest

# Verbose output
pytest -v

# Show print statements
pytest -s

# Run specific test file
pytest test_math.py

# Run specific test function
pytest test_math.py::test_add

2. Test Classes for Organization

# test_calculator.py
class Calculator:
    def add(self, a, b):
        return a + b

    def multiply(self, a, b):
        return a * b

class TestCalculator:
    def test_add(self):
        calc = Calculator()
        assert calc.add(2, 3) == 5

    def test_multiply(self):
        calc = Calculator()
        assert calc.multiply(4, 5) == 20

    def test_add_negative(self):
        calc = Calculator()
        assert calc.add(-1, -1) == -2

3. Assertions and Expected Failures

import pytest

# Test exception raising
def divide(a, b):
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

def test_divide_by_zero():
    with pytest.raises(ValueError, match="Cannot divide by zero"):
        divide(10, 0)

def test_divide_success():
    assert divide(10, 2) == 5.0

# Test approximate equality
def test_float_comparison():
    assert 0.1 + 0.2 == pytest.approx(0.3)

# Test containment
def test_list_contains():
    result = [1, 2, 3, 4]
    assert 3 in result
    assert len(result) == 4

Fixtures - Dependency Injection

Basic Fixtures

# conftest.py
import pytest

@pytest.fixture
def sample_data():
    """Provide sample data for tests."""
    return {"name": "Alice", "age": 30, "email": "[email protected]"}

@pytest.fixture
def empty_list():
    """Provide an empty list."""
    return []

# test_fixtures.py
def test_sample_data(sample_data):
    assert sample_data["name"] == "Alice"
    assert sample_data["age"] == 30

def test_empty_list(empty_list):
    empty_list.append(1)
    assert len(empty_list) == 1

Fixture Scopes

import pytest

# Function scope (default) - runs for each test
@pytest.fixture(scope="function")
def user():
    return {"id": 1, "name": "Alice"}

# Class scope - runs once per test class
@pytest.fixture(scope="class")
def database():
    db = setup_database()
    yield db
    db.close()

# Module scope - runs once per test module
@pytest.fixture(scope="module")
def api_client():
    client = APIClient()
    yield client
    client.shutdown()

# Session scope - runs once for entire test session
@pytest.fixture(scope="session")
def app_config():
    return load_config()

Fixture Setup and Teardown

import pytest
import tempfile
import shutil

@pytest.fixture
def temp_directory():
    """Create a temporary directory for test."""
    temp_dir = tempfile.mkdtemp()