python-testing

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SKILL.md (first lines)

---
name: python-testing
description: Python testing strategies using pytest, TDD methodology, fixtures, mocking, parametrization, and coverage requirements.
---

# Python Testing Patterns

Comprehensive testing strategies for Python applications using pytest, TDD methodology, and best practices.

## When to Activate

- Writing new Python code (follow TDD: red, green, refactor)
- Designing test suites for Python projects
- Reviewing Python test coverage
- Setting up testing infrastructure

## Core Testing Philosophy

### Test-Driven Development (TDD)

Always follow the TDD cycle:

1. **RED**: Write a failing test for the desired behavior
2. **GREEN**: Write minimal code to make the test pass
3. **REFACTOR**: Improve code while keeping tests green

```python
# Step 1: Write failing test (RED)
def test_add_numbers():
    result = add(2, 3)
    assert result == 5

# Step 2: Write minimal implementation (GREEN)
def add(a, b):
    return a + b

# Step 3: Refactor if needed (REFACTOR)
```

### Coverage Requirements

- **Target**: 80%+ code coverage
- **Critical paths**: 100% coverage required
- Use `pytest --cov` to measure coverage

```bash
pytest --cov=mypackage --cov-report=term-missing --cov-report=html
```

## pytest Fundamentals

### Basic Test Structure

```python
import pytest

def test_addition():
    """Test basic addition."""
    assert 2 + 2 == 4

def test_string_uppercase():
    """Test string uppercasing."""
    text = "hello"
    assert text.upper() == "HELLO"

def test_list_append():
    """Test list append."""
    items = [1, 2, 3]
    items.append(4)
    assert 4 in items
    assert len(items) == 4
```

### Assertions

```python
# Equality
assert result == expected

# Inequality
assert result != unexpected

# Truthiness
assert result  # Truthy
assert not result  # Falsy
assert result is True  # Exactly True
assert result is False  # Exactly False
assert result is None  # Exactly None

# Membership
assert item in collection
assert item not in collection

# Comparisons
assert result > 0
assert 0 <= result <= 100

# Type checking
assert isinstance(result, str)

# Exception testing (preferred approach)
with pytest.raises(ValueError):
    raise ValueError("error message")

# Check exception message
with pytest.raises(ValueError, match="invalid input"):
    raise ValueError("invalid input provided")

# Check exception attributes
with pytest.raises(ValueError) as exc_info:
    raise ValueError("error message")
assert str(exc_info.value) == "error message"
```

## Fixtures

### Basic Fixture Usage

```python
import pytest

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