# Python Testing Patterns

> |

- **Type:** Skill
- **Install:** `agentstack add skill-jh941213-codex-lattice-python-testing-patterns`
- **Verified:** Yes — security-reviewed for prompt injection and unsafe behavior
- **Seller:** [jh941213](https://agentstack.voostack.com/s/jh941213)
- **Installs:** 0
- **Category:** [Agent Skills](https://agentstack.voostack.com/c/agent-skills)
- **Latest version:** 0.1.0
- **License:** MIT
- **Upstream author:** [jh941213](https://github.com/jh941213)
- **Source:** https://github.com/jh941213/codex-lattice/tree/main/skills/python-testing-patterns

## Install

```sh
agentstack add skill-jh941213-codex-lattice-python-testing-patterns
```

Requires the [AgentStack CLI](https://agentstack.voostack.com/docs/cli). Works with Claude Code, Cursor, and any MCP-compatible agent.

## About

# 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]  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).

…

## Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

- **Author:** [jh941213](https://github.com/jh941213)
- **Source:** [jh941213/codex-lattice](https://github.com/jh941213/codex-lattice)
- **License:** MIT

Install and usage instructions live in the source repository linked above.

## Pricing

- **Free** — Free

## Security capabilities

Automated source analysis of v0.1.0 — what this tool can access:

- **Network access:** yes
- **Filesystem access:** yes
- **Shell / process execution:** no
- **Environment & secrets:** yes
- **Dynamic code execution:** no

*"Yes" means the capability is present in the source — more access means more to trust, not that it is unsafe.*


## Versions

- **0.1.0** — security scan: passed — Imported from the upstream source.

## Links

- Listing page: https://agentstack.voostack.com/l/skill-jh941213-codex-lattice-python-testing-patterns
- Seller: https://agentstack.voostack.com/s/jh941213
- Browse the marketplace: https://agentstack.voostack.com/browse

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