Pytest Testing
Overview
Master software testing with pytest, Python's most popular testing framework. Learn test-driven development (TDD), write maintainable tests, and ensure code quality through comprehensive testing strategies.
Learning Objectives
- Write unit, integration, and functional tests with pytest
- Use fixtures for test setup and teardown
- Mock external dependencies effectively
- Implement test-driven development (TDD)
- Measure and improve code coverage
- Integrate tests with CI/CD pipelines
Core Topics
1. Pytest Basics
- Test discovery and naming conventions
- Assertions and comparison
- Test organization (files, classes, modules)
- Running tests (command-line options)
- Markers and test selection
- Parametrized tests
Code Example:
# test_calculator.py
import pytest
def add(a, b):
return a + b
def divide(a, b):
if b == 0:
raise ValueError("Cannot divide by zero")
return a / b
# Basic test
def test_add():
assert add(2, 3) == 5
assert add(-1, 1) == 0
assert add(0, 0) == 0
# Test exceptions
def test_divide_by_zero():
with pytest.raises(ValueError, match="Cannot divide by zero"):
divide(10, 0)
# Parametrized test
@pytest.mark.parametrize("a,b,expected", [
(10, 2, 5),
(20, 4, 5),
(100, 10, 10),
(-10, 2, -5),
])
def test_divide(a, b, expected):
assert divide(a, b) == expected
# Test with marker
@pytest.mark.slow
def test_complex_operation():
# This test takes a long time
result = sum(range(1000000))
assert result == 4999995000002. Fixtures & Test Setup
- Fixture scopes (function, class, module, session)
- Fixture dependencies
- Parametrized fixtures
- Built-in fixtures (tmpdir, capsys, monkeypatch)
- conftest.py for shared fixtures
Code Example:
# conftest.py
import pytest
import tempfile
from pathlib import Path
@pytest.fixture
def sample_data():
"""Provide sample data for tests"""
return {
'users': [
{'id': 1, 'name': 'Alice', 'email': 'alice@example.com'},
{'id': 2, 'name': 'Bob', 'email': 'bob@example.com'},
]
}
@pytest.fixture
def temp_file():
"""Create temporary file for testing"""
with tempfile.NamedTemporaryFile(mode='w', delete=False) as f:
f.write("Test data")
temp_path = f.name
yield temp_path
# Cleanup
Path(temp_path).unlink()
@pytest.fixture(scope='module')
def database_connection():
"""Module-scoped database connection"""
db = DatabaseConnection('test.db')
db.connect()
yield db
db.close()
# test_users.py
def test_user_count(sample_data):
assert len(sample_data['users']) == 2
def test_user_names(sample_data):
names = [user['name'] for user in sample_data['users']]
assert 'Alice' in names
assert 'Bob' in names
def test_file_operations(temp_file):
content = Path(temp_file).read_text()
assert content == "Test data"3. Mocking & Test Doubles
- unittest.mock basics
- Mocking functions and methods
- Patching objects
- Mock assertions
- Side effects and return values
- Testing with external dependencies
Code Example:
# api_client.py
import requests
class APIClient:
def __init__(self, base_url):
self.base_url = base_url
def get_user(self, user_id):
response = requests.get(f"{self.base_url}/users/{user_id}")
response.raise_for_status()
return response.json()
def create_user(self, user_data):
response = requests.post(f"{self.base_url}/users", json=user_data)
response.raise_for_status()
return response.json()
# test_api_client.py
from unittest.mock import Mock, patch
import pytest
@patch('api_client.requests.get')
def test_get_user(mock_get):
# Setup mock
mock_response = Mock()
mock_response.json.return_value = {'id': 1, 'name': 'Alice'}
mock_response.raise_for_status.return_value = None
mock_get.return_value = mock_response
# Test
client = APIClient('https://api.example.com')
user = client.get_user(1)
# Assertions
assert user['name'] == 'Alice'
mock_get.assert_called_once_with('https://api.example.com/users/1')
@patch('api_client.requests.post')
def test_create_user(mock_post):
# Setup mock
mock_response = Mock()
mock_response.json.return_value = {'id': 3, 'name': 'Charlie'}
mock_post.return_value = mock_response
# Test
client = APIClient('https://api.example.com')
user_data = {'name': 'Charlie', 'email': 'charlie@example.com'}
result = client.create_user(user_data)
# Assertions
assert result['id'] == 3
mock_post.assert_called_once_with(
'https://api.example.com/users',
json=user_data
)4. Coverage & CI/CD Integration
- Measuring code coverage with pytest-cov
- Coverage reports (terminal, HTML, XML)
- Setting coverage thresholds
- GitHub Actions integration
- GitLab CI integration
- Pre-commit hooks
Code Example:
# pytest.ini
[tool:pytest]
testpaths = tests
python_files = test_*.py
python_classes = Test*
python_functions = test_*
addopts =
--cov=myapp
--cov-report=html
--cov-report=term-missing
--cov-fail-under=80
-v
# .github/workflows/test.yml
name: Tests
on: [push, pull_request]
jobs:
test:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v3
- name: Set up Python
uses: actions/setup-python@v4
with:
python-version: '3.11'
- name: Install dependencies
run: |
pip install -r requirements.txt
pip install pytest pytest-cov
- name: Run tests
run: |
pytest --cov=myapp --cov-report=xml
- name: Upload coverage
uses: codecov/codecov-action@v3
with:
file: ./coverage.xml
# Command line usage
# Run all tests
pytest
# Run with coverage
pytest --cov=myapp
# Generate HTML coverage report
pytest --cov=myapp --cov-report=html
# Run specific test file
pytest tests/test_api.py
# Run tests with marker
pytest -m slow
# Run tests with verbose output
pytest -v
# Stop on first failure
pytest -xHands-On Practice
Project 1: Calculator TDD
Build a calculator using test-driven development.
Requirements:
- Write tests BEFORE implementation
- Basic operations (add, subtract, multiply, divide)
- Error handling (division by zero)
- Scientific operations (power, sqrt, log)
- Test coverage > 90%
Key Skills: TDD workflow, parametrized tests, exception testing
Project 2: API Testing Suite
Create comprehensive test suite for a REST API.
Requirements:
- Mock HTTP requests
- Test CRUD operations
- Error handling tests
- Authentication tests
- Integration tests
- CI/CD pipeline setup
Key Skills: Mocking, fixtures, integration testing
Project 3: Database Testing
Test database operations with fixtures and transactions.
Requirements:
- Setup test database fixture
- Test CRUD operations
- Transaction rollback
- Data validation
- Performance tests
- Coverage report
Key Skills: Database fixtures, cleanup, performance testing
Assessment Criteria
- Write clear, maintainable tests
- Use fixtures appropriately
- Mock external dependencies effectively
- Achieve >80% code coverage
- Follow TDD principles
- Integrate tests with CI/CD
- Write meaningful assertions
Resources
Official Documentation
- Pytest Docs - Official documentation
- pytest-cov - Coverage plugin
- unittest.mock - Mocking library
Learning Platforms
- Test-Driven Development with Python - TDD book
- Python Testing with pytest - Brian Okken's book
- Real Python Testing - Tutorials
Tools
- pytest-xdist - Parallel testing
- pytest-mock - Mocking helper
- Hypothesis - Property-based testing
- tox - Testing automation
Next Steps
After mastering pytest, explore:
- Property-based testing - Hypothesis library
- Performance testing - pytest-benchmark
- Mutation testing - mutmut
- Load testing - Locust, pytest-load