majiayu000 Backend Python Testing
Comprehensive guide to implementing robust testing strategies in Python using pytest, fixtures, mocking, parameterization, and test-driven development practices.
When to Use This Skill
- Writing unit tests for Python code
- Setting up test suites and test infrastructure
- Implementing test-driven development (TDD)
- Creating integration tests for APIs and services
- Mocking external dependencies and services
- Testing async code and concurrent operations
- Setting up continuous testing in CI/CD
- Implementing property-based testing
- Testing database operations
- Debugging failing tests
Core Concepts
1. Test Types
- Unit Tests: Test individual functions/classes in isolation
- Integration Tests: Test interaction between components
- Functional Tests: Test complete features end-to-end
- Performance Tests: Measure speed and resource usage
2. Test Structure (AAA Pattern)
- Arrange: Set up test data and preconditions
- Act: Execute the code under test
- Assert: Verify the results
3. Test Coverage
- Measure what code is exercised by tests
- Identify untested code paths
- Aim for meaningful coverage, not just high percentages
4. Test Isolation
- Tests should be independent
- No shared state between tests
- Each test should clean up after itself
Quick Start
# test_example.py
def add(a, b):
return a + b
def test_add():
"""Basic test example."""
result = add(2, 3)
assert result == 5
def test_add_negative():
"""Test with negative numbers."""
assert add(-1, 1) == 0
# Run with: pytest test_example.py
Fundamental Patterns
Pattern 1: Basic pytest Tests
# test_calculator.py
import pytest
class Calculator:
"""Simple calculator for testing."""
def add(self, a: float, b: float) -> float:
return a + b
def subtract(self, a: float, b: float) -> float:
return a - b
def multiply(self, a: float, b: float) -> float:
return a * b
def divide(self, a: float, b: float) -> float:
if b == 0:
raise ValueError("Cannot divide by zero")
return a / b
def test_addition():
"""Test addition."""
calc = Calculator()
assert calc.add(2, 3) == 5
assert calc.add(-1, 1) == 0
assert calc.add(0, 0) == 0
def test_subtraction():
"""Test subtraction."""
calc = Calculator()
assert calc.subtract(5, 3) == 2
assert calc.subtract(0, 5) == -5
def test_multiplication():
"""Test multiplication."""
calc = Calculator()
assert calc.multiply(3, 4) == 12
assert calc.multiply(0, 5) == 0
def test_division():
"""Test division."""
calc = Calculator()
assert calc.divide(6, 3) == 2
assert calc.divide(5, 2) == 2.5
def test_division_by_zero():
"""Test division by zero raises error."""
calc = Calculator()
with pytest.raises(ValueError, match="Cannot divide by zero"):
calc.divide(5, 0)
Pattern 2: Fixtures for Setup and Teardown
# test_database.py
import pytest
from typing import Generator
class Database:
"""Simple database class."""
def __init__(self, connection_string: str):
self.connection_string = connection_string
self.connected = False
def connect(self):
"""Connect to database."""
self.connected = True
def disconnect(self):
"""Disconnect from database."""
self.connected = False
def query(self, sql: str) -> list:
"""Execute query."""
if not self.connected:
raise RuntimeError("Not connected")
return [{"id": 1, "name": "Test"}]
@pytest.fixture
def db() -> Generator[Database, None, None]:
"""Fixture that provides connected database."""
# Setup
database = Database("sqlite:///:memory:")
database.connect()
# Provide to test
yield database
# Teardown
database.disconnect()
def test_database_query(db):
"""Test database query with fixture."""
results = db.query("SELECT * FROM users")
assert len(results) == 1
assert results[0]["name"] == "Test"
@pytest.fixture(scope="session")
def app_config():
"""Session-scoped fixture - created once per test session."""
return {
"database_url": "postgresql://localhost/test",
"api_key": "test-key",
"debug": True
}
@pytest.fixture(scope="module")
def api_client(app_config):
"""Module-scoped fixture - created once per test module."""
# Setup expensive resource
client = {"config": app_config, "session": "active"}
yield client
# Cleanup
client["session"] = "closed"
def test_api_client(api_client):
"""Test using api client fixture."""
assert api_client["session"] == "active"
assert api_client["config"]["debug"] is True
Pattern 3: Parameterized Tests
# test_validation.py
import pytest
def is_valid_email(email: str) -> bool:
"""Check if email is valid."""
return "@" in email and "." in email.split("@")[1]
@pytest.mark.parametrize("email,expected", [
("user@example.com", True),
("test.user@domain.co.uk", True),
("invalid.email", False),
("@example.com", False),
("user@domain", False),
("", False),
])
def test_email_validation(email, expected):
"""Test email validation with various inputs."""
assert is_valid_email(email) == expected
@pytest.mark.parametrize("a,b,expected", [
(2, 3, 5),
(0, 0, 0),
(-1, 1, 0),
(100, 200, 300),
(-5, -5, -10),
])
def test_addition_parameterized(a, b, expected):
"""Test addition with multiple parameter sets."""
from test_calculator import Calculator
calc = Calculator()
assert calc.add(a, b) == expected
# Using pytest.param for special cases
@pytest.mark.parametrize("value,expected", [
pytest.param(1, True, id="positive"),
pytest.param(0, False, id="zero"),
pytest.param(-1, False, id="negative"),
])
def test_is_positive(value, expected):
"""Test with custom test IDs."""
assert (value > 0) == expected
Pattern 4: Mocking with unittest.mock
# test_api_client.py
import pytest
from unittest.mock import Mock, patch, MagicMock
import requests
class APIClient:
"""Simple API client."""
def __init__(self, base_url: str):
self.base_url = base_url
def get_user(self, user_id: int) -> dict:
"""Fetch user from API."""
response = requests.get(f"{self.base_url}/users/{user_id}")
response.raise_for_status()
return response.json()
def create_user(self, data: dict) -> dict:
"""Create new user."""
response = requests.post(f"{self.base_url}/users", json=data)
response.raise_for_status()
return response.json()
def test_get_user_success():
"""Test successful API call with mock."""
client = APIClient("https://api.example.com")
mock_response = Mock()
mock_response.json.return_value = {"id": 1, "name": "John Doe"}
mock_response.raise_for_status.return_value = None
with patch("requests.get", return_value=mock_response) as mock_get:
user = client.get_user(1)
assert user["id"] == 1
assert user["name"] == "John Doe"
mock_get.assert_called_once_with("https://api.example.com/users/1")
def test_get_user_not_found():
"""Test API call with 404 error."""
client = APIClient("https://api.example.com")
mock_response = Mock()
mock_response.raise_for_status.side_effect = requests.HTTPError("404 Not Found")
with patch("requests.get", return_value=mock_response):
with pytest.raises(requests.HTTPError):
client.get_user(999)
@patch("requests.post")
def test_create_user(mock_post):
"""Test user creation with decorator syntax."""
client = APIClient("https://api.example.com")
mock_post.return_value.json.return_value = {"id": 2, "name": "Jane Doe"}
mock_post.return_value.raise_for_status.return_value = None
user_data = {"name": "Jane Doe", "email": "jane@example.com"}
result = client.create_user(user_data)
assert result["id"] == 2
mock_post.assert_called_once()
call_args = mock_post.call_args
assert call_args.kwargs["json"] == user_data
Pattern 5: Testing Exceptions
# test_exceptions.py
import pytest
def divide(a: float, b: float) -> float:
"""Divide a by b."""
if b == 0:
raise ZeroDivisionError("Division by zero")
if not isinstance(a, (int, float)) or not isinstance(b, (int, float)):
raise TypeError("Arguments must be numbers")
return a / b
def test_zero_division():
"""Test exception is raised for division by zero."""
with pytest.raises(ZeroDivisionError):
divide(10, 0)
def test_zero_division_with_message():
"""Test exception message."""
with pytest.raises(ZeroDivisionError, match="Division by zero"):
divide(5, 0)
def test_type_error():
"""Test type error exception."""
with pytest.raises(TypeError, match="must be numbers"):
divide("10", 5)
def test_exception_info():
"""Test accessing exception info."""
with pytest.raises(ValueError) as exc_info:
int("not a number")
assert "invalid literal" in str(exc_info.value)
Advanced Patterns
Pattern 6: Testing Async Code
# test_async.py
import pytest
import asyncio
async def fetch_data(url: str) -> dict:
"""Fetch data asynchronously."""
await asyncio.sleep(0.1)
return {"url": url, "data": "result"}
@pytest.mark.asyncio
async def test_fetch_data():
"""Test async function."""
result = await fetch_data("https://api.example.com")
assert result["url"] == "https://api.example.com"
assert "data" in result
@pytest.mark.asyncio
async def test_concurrent_fetches():
"""Test concurrent async operations."""
urls = ["url1", "url2", "url3"]
tasks = [fetch_data(url) for url in urls]
results = await asyncio.gather(*tasks)
assert len(results) == 3
assert all("data" in r for r in results)
@pytest.fixture
async def async_client():
"""Async fixture."""
client = {"connected": True}
yield client
client["connected"] = False
@pytest.mark.asyncio
async def test_with_async_fixture(async_client):
"""Test using async fixture."""
assert async_client["connected"] is True
Pattern 7: Monkeypatch for Testing
# test_environment.py
import os
import pytest
def get_database_url() -> str:
"""Get database URL from environment."""
return os.environ.get("DATABASE_URL", "sqlite:///:memory:")
def test_database_url_default():
"""Test default database URL."""
# Will use actual environment variable if set
url = get_database_url()
assert url
def test_database_url_custom(monkeypatch):
"""Test custom database URL with monkeypatch."""
monkeypatch.setenv("DATABASE_URL", "postgresql://localhost/test")
assert get_database_url() == "postgresql://localhost/test"
def test_database_url_not_set(monkeypatch):
"""Test when env var is not set."""
monkeypatch.delenv("DATABASE_URL", raising=False)
assert get_database_url() == "sqlite:///:memory:"
class Config:
"""Configuration class."""
def __init__(self):
self.api_key = "production-key"
def get_api_key(self):
return self.api_key
def test_monkeypatch_attribute(monkeypatch):
"""Test monkeypatching object attributes."""
config = Config()
monkeypatch.setattr(config, "api_key", "test-key")
assert config.get_api_key() == "test-key"
Pattern 8: Temporary Files and Directories
# test_file_operations.py
import pytest
from pathlib import Path
def save_data(filepath: Path, data: str):
"""Save data to file."""
filepath.write_text(data)
def load_data(filepath: Path) -> str:
"""Load data from file."""
return filepath.read_text()
def test_file_operations(tmp_path):
"""Test file operations with temporary directory."""
# tmp_path is a pathlib.Path object
test_file = tmp_path / "test_data.txt"
# Save data
save_data(test_file, "Hello, World!")
# Verify file exists
assert test_file.exists()
# Load and verify data
data = load_data(test_file)
assert data == "Hello, World!"
def test_multiple_files(tmp_path):
"""Test with multiple temporary files."""
files = {
"file1.txt": "Content 1",
"file2.txt": "Content 2",
"file3.txt": "Content 3"
}
for filename, content in files.items():
filepath = tmp_path / filename
save_data(filepath, content)
# Verify all files created
assert len(list(tmp_path.iterdir())) == 3
# Verify contents
for filename, expected_content in files.items():
filepath = tmp_path / filename
assert load_data(filepath) == expected_content
Pattern 9: Custom Fixtures and Conftest
# conftest.py
"""Shared fixtures for all tests."""
import pytest
@pytest.fixture(scope="session")
def database_url():
"""Provide database URL for all tests."""
return "postgresql://localhost/test_db"
@pytest.fixture(autouse=True)
def reset_database(database_url):
"""Auto-use fixture that runs before each test."""
# Setup: Clear database
print(f"Clearing database: {database_url}")
yield
# Teardown: Clean up
print("Test completed")
@pytest.fixture
def sample_user():
"""Provide sample user data."""
return {
"id": 1,
"name": "Test User",
"email": "test@example.com"
}
@pytest.fixture
def sample_users():
"""Provide list of sample users."""
return [
{"id": 1, "name": "User 1"},
{"id": 2, "name": "User 2"},
{"id": 3, "name": "User 3"},
]
# Parametrized fixture
@pytest.fixture(params=["sqlite", "postgresql", "mysql"])
def db_backend(request):
"""Fixture that runs tests with different database backends."""
return request.param
def test_with_db_backend(db_backend):
"""This test will run 3 times with different backends."""
print(f"Testing with {db_backend}")
assert db_backend in ["sqlite", "postgresql", "mysql"]
Pattern 10: Property-Based Testing
# test_properties.py
from hypothesis import given, strategies as st
import pytest
def reverse_string(s: str) -> str:
"""Reverse a string."""
return s[::-1]
@given(st.text())
def test_reverse_twice_is_original(s):
"""Property: reversing twice returns original."""
assert reverse_string(reverse_string(s)) == s
@given(st.text())
def test_reverse_length(s):
"""Property: reversed string has same length."""
assert len(reverse_string(s)) == len(s)
@given(st.integers(), st.integers())
def test_addition_commutative(a, b):
"""Property: addition is commutative."""
assert a + b == b + a
@given(st.lists(st.integers()))
def test_sorted_list_properties(lst):
"""Property: sorted list is ordered."""
sorted_lst = sorted(lst)
# Same length
assert len(sorted_lst) == len(lst)
# All elements present
assert set(sorted_lst) == set(lst)
# Is ordered
for i in range(len(sorted_lst) - 1):
assert sorted_lst[i] <= sorted_lst[i + 1]
Testing Best Practices
Test Organization
# tests/
# __init__.py
# conftest.py # Shared fixtures
# test_unit/ # Unit tests
# test_models.py
# test_utils.py
# test_integration/ # Integration tests
# test_api.py
# test_database.py
# test_e2e/ # End-to-end tests
# test_workflows.py
Test Naming
# Good test names
def test_user_creation_with_valid_data():
"""Clear name describes what is being tested."""
pass
def test_login_fails_with_invalid_password():
"""Name describes expected behavior."""
pass
def test_api_returns_404_for_missing_resource():
"""Specific about inputs and expected outcomes."""
pass
# Bad test names
def test_1(): # Not descriptive
pass
def test_user(): # Too vague
pass
def test_function(): # Doesn't explain what's tested
pass
Test Markers
# test_markers.py
import pytest
@pytest.mark.slow
def test_slow_operation():
"""Mark slow tests."""
import time
time.sleep(2)
@pytest.mark.integration
def test_database_integration():
"""Mark integration tests."""
pass
@pytest.mark.skip(reason="Feature not implemented yet")
def test_future_feature():
"""Skip tests temporarily."""
pass
@pytest.mark.skipif(os.name == "nt", reason="Unix only test")
def test_unix_specific():
"""Conditional skip."""
pass
@pytest.mark.xfail(reason="Known bug #123")
def test_known_bug():
"""Mark expected failures."""
assert False
# Run with:
# pytest -m slow # Run only slow tests
# pytest -m "not slow" # Skip slow tests
# pytest -m integration # Run integration tests
Coverage Reporting
# Install coverage
pip install pytest-cov
# Run tests with coverage
pytest --cov=myapp tests/
# Generate HTML report
pytest --cov=myapp --cov-report=html tests/
# Fail if coverage below threshold
pytest --cov=myapp --cov-fail-under=80 tests/
# Show missing lines
pytest --cov=myapp --cov-report=term-missing tests/
Testing Database Code
# test_database_models.py
import pytest
from sqlalchemy import create_engine, Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker, Session
Base = declarative_base()
class User(Base):
"""User model."""
__tablename__ = "users"
id = Column(Integer, primary_key=True)
name = Column(String(50))
email = Column(String(100), unique=True)
@pytest.fixture(scope="function")
def db_session() -> Session:
"""Create in-memory database for testing."""
engine = create_engine("sqlite:///:memory:")
Base.metadata.create_all(engine)
SessionLocal = sessionmaker(bind=engine)
session = SessionLocal()
yield session
session.close()
def test_create_user(db_session):
"""Test creating a user."""
user = User(name="Test User", email="test@example.com")
db_session.add(user)
db_session.commit()
assert user.id is not None
assert user.name == "Test User"
def test_query_user(db_session):
"""Test querying users."""
user1 = User(name="User 1", email="user1@example.com")
user2 = User(name="User 2", email="user2@example.com")
db_session.add_all([user1, user2])
db_session.commit()
users = db_session.query(User).all()
assert len(users) == 2
def test_unique_email_constraint(db_session):
"""Test unique email constraint."""
from sqlalchemy.exc import IntegrityError
user1 = User(name="User 1", email="same@example.com")
user2 = User(name="User 2", email="same@example.com")
db_session.add(user1)
db_session.commit()
db_session.add(user2)
with pytest.raises(IntegrityError):
db_session.commit()
CI/CD Integration
# .github/workflows/test.yml
name: Tests
on: [push, pull_request]
jobs:
test:
runs-on: ubuntu-latest
strategy:
matrix:
python-version: ["3.9", "3.10", "3.11", "3.12"]
steps:
- uses: actions/checkout@v3
- name: Set up Python
uses: actions/setup-python@v4
with:
python-version: ${{ matrix.python-version }}
- name: Install dependencies
run: |
pip install -e ".[dev]"
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
Configuration Files
# pytest.ini
[pytest]
testpaths = tests
python_files = test_*.py
python_classes = Test*
python_functions = test_*
addopts =
-v
--strict-markers
--tb=short
--cov=myapp
--cov-report=term-missing
markers =
slow: marks tests as slow
integration: marks integration tests
unit: marks unit tests
e2e: marks end-to-end tests
# pyproject.toml
[tool.pytest.ini_options]
testpaths = ["tests"]
python_files = ["test_*.py"]
addopts = [
"-v",
"--cov=myapp",
"--cov-report=term-missing",
]
[tool.coverage.run]
source = ["myapp"]
omit = ["*/tests/*", "*/migrations/*"]
[tool.coverage.report]
exclude_lines = [
"pragma: no cover",
"def __repr__",
"raise AssertionError",
"raise NotImplementedError",
]
Resources
- pytest documentation: https://docs.pytest.org/
- unittest.mock: https://docs.python.org/3/library/unittest.mock.html
- hypothesis: Property-based testing
- pytest-asyncio: Testing async code
- pytest-cov: Coverage reporting
- pytest-mock: pytest wrapper for mock
Best Practices Summary
- Write tests first (TDD) or alongside code
- One assertion per test when possible
- Use descriptive test names that explain behavior
- Keep tests independent and isolated
- Use fixtures for setup and teardown
- Mock external dependencies appropriately
- Parametrize tests to reduce duplication
- Test edge cases and error conditions
- Measure coverage but focus on quality
- Run tests in CI/CD on every commit