python-testing

Python Testing Patterns

Use the project's installed version and conventions. Verify version-sensitive APIs, security settings, and deployment support with documentation-lookup before applying examples; these patterns are reference guidance, not a tested guarantee for every current release. Do not replace an existing toolchain solely to match an example.

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

# 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

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

pytest Fundamentals

Basic Test Structure

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

# 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"