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python-testing-patterns

Implement comprehensive testing strategies with pytest, fixtures,

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# Python Testing Patterns

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

```python
# 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

```python
# 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

```python
# 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

```python
# 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

```python
# 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

```python
# 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

```python
# 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

```python
# 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

```python
# 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

```python
# 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

```python
# 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]
```

## Test Design Principles

### One Behavior Per Test

Each test should verify exactly one behavior. This makes failures easy to diagnose and tests easy to maintain.

```python
# BAD - testing multiple behaviors
def test_user_service():
    user = service.create_user(data)
    assert user.id is not None
    assert user.email == data["email"]
    updated = service.update_user(user.id, {"name": "New"})
    assert updated.name == "New"

# GOOD - focused tests
def test_create_user_assigns_id():
    user = service.create_user(data)
    assert user.id is not None

def test_create_user_stores_email():
    user = service.create_user(data)
    assert user.email == data["email"]

def test_update_user_changes_name():
    user = service.create_user(data)
    updated = service.update_user(user.id, {"name": "New"})
    assert updated.name == "New"
```

### Test Error Paths

Always test failure cases, not just happy paths.

```python
def test_get_user_raises_not_found():
    with pytest.raises(UserNotFoundError) as exc_info:
        service.get_user("nonexistent-id")

assert "nonexistent-id" in str(exc_info.value)

def test_create_user_rejects_invalid_email():
    with pytest.raises(ValueError, match="Invalid email format"):
        service.create_user({"email": "not-an-email"})
```

## Testing Best Practices

### Test Organization

```python
# 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 Convention

A common pattern: `test_<unit>_<scenario>_<expected_outcome>`. Adapt to your team's preferences.

```python
# Pattern: test_<unit>_<scenario>_<expected>
def test_create_user_with_valid_data_returns_user():
    ...

def test_create_user_with_duplicate_email_raises_conflict():
    ...

def test_get_user_with_unknown_id_returns_none():
    ...

# Good test names - clear and descriptive
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 - avoid these
def test_1():  # Not descriptive
    pass

def test_user():  # Too vague
    pass

def test_function():  # Doesn't explain what's tested
    pass
```

### Testing Retry Behavior

Verify that retry logic works correctly using mock side effects.

```python
from unittest.mock import Mock

def test_retries_on_transient_error():
    """Test that service retries on transient failures."""
    client = Mock()
    # Fail twice, then succeed
    client.request.side_effect = [
        ConnectionError("Failed"),
        ConnectionError("Failed"),
        {"status": "ok"},
    ]

service = ServiceWithRetry(client, max_retries=3)
    result = service.fetch()

assert result == {"status": "ok"}
    assert client.request.call_count == 3

def test_gives_up_after_max_retries():
    """Test that service stops retrying after max attempts."""
    client = Mock()
    client.request.side_effect = ConnectionError("Failed")

service = ServiceWithRetry(client, max_retries=3)

with pytest.raises(ConnectionError):
        service.fetch()

assert client.request.call_count == 3

def test_does_not_retry_on_permanent_error():
    """Test that permanent errors are not retried."""
    client = Mock()
    client.request.side_effect = ValueError("Invalid input")

service = ServiceWithRetry(client, max_retries=3)

with pytest.raises(ValueError):
        service.fetch()

# Only called once - no retry for ValueError
    assert client.request.call_count == 1
```

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