Testing Best Practices

Test-driven development, mocking patterns, and test organization.

Overview

Master testing best practices.

Test-Driven Development

# Step 1: Write failing test
def test_add_positive_numbers():
    assert add(2, 3) == 5

# Step 2: Write minimal code to pass
def add(a, b):
    return a + b

# Step 3: Refactor if needed
# Step 4: Repeat

Mocking Patterns

from unittest.mock import Mock, patch, MagicMock
from datetime import datetime

# Mock external service
@patch('myapp.services.external_api')
def test_api_call(mock_api):
    mock_api.get.return_value.json.return_value = {"status": "ok"}
    
    result = fetch_data()
    assert result["status"] == "ok"
    mock_api.get.assert_called_once()

# Mock time-dependent code
@patch('myapp.utils.datetime')
def test_timestamp(mock_dt):
    mock_dt.now.return_value = datetime(2024, 1, 1)
    timestamp = get_timestamp()
    assert timestamp == "2024-01-01"

Test Organization

# tests/test_user_service.py
import pytest
from myapp.services import UserService
from myapp.models import User

class TestUserService:
    @pytest.fixture
    def service(self):
        return UserService()
    
    @pytest.fixture
    def sample_user(self):
        return User(name="Alice", email="alice@test.com")
    
    def test_create_user(self, service, sample_user):
        result = service.create(sample_user)
        assert result.id is not None
    
    def test_find_user(self, service, sample_user):
        service.create(sample_user)
        found = service.find_by_email("alice@test.com")
        assert found.name == "Alice"
    
    def test_user_not_found(self, service):
        with pytest.raises(UserNotFoundError):
            service.find_by_email("unknown@test.com")

Practice

Write tests for a complex service with dependencies.