            /\
           /E2E\       Few, slow, expensive
          /────\
         /Integration\  Some, medium speed
        /────────────\
       /  Unit Tests  \ Many, fast, cheap
      /────────────────\

Type	Coverage	Speed	Cost
Unit	80%+	Fast	Low
Integration	60%+	Medium	Medium
E2E	Critical paths	Slow	High

Examples

Unit Test (pytest)

import pytest
from unittest.mock import Mock
from app.services.user_service import UserService

class TestUserService:
    @pytest.fixture
    def service(self):
        return UserService(db=Mock())

    def test_get_user_returns_user(self, service):
        service.db.get_user.return_value = {"id": 1, "name": "John"}

        result = service.get_user(1)

        assert result["name"] == "John"
        service.db.get_user.assert_called_once_with(1)

    def test_get_user_raises_when_not_found(self, service):
        service.db.get_user.return_value = None

        with pytest.raises(UserNotFoundError):
            service.get_user(999)

Integration Test

import pytest
from fastapi.testclient import TestClient
from app.main import app
from app.database import get_db

@pytest.fixture
def client(test_db):
    def override_get_db():
        return test_db
    app.dependency_overrides[get_db] = override_get_db
    with TestClient(app) as c:
        yield c

def test_create_and_get_user(client):
    # Create user
    response = client.post("/users", json={"email": "test@example.com"})
    assert response.status_code == 201
    user_id = response.json()["id"]

    # Get user
    response = client.get(f"/users/{user_id}")
    assert response.status_code == 200
    assert response.json()["email"] == "test@example.com"

Load Test (k6)

import http from 'k6/http';
import { check, sleep } from 'k6';

export const options = {
  stages: [
    { duration: '30s', target: 100 },
    { duration: '1m', target: 100 },
    { duration: '30s', target: 0 },
  ],
  thresholds: {
    http_req_duration: ['p(95)<200'],
    http_req_failed: ['rate<0.01'],
  },
};

export default function () {
  const res = http.get('http://localhost:8000/api/v1/users');
  check(res, {
    'status is 200': (r) => r.status === 200,
    'response time < 200ms': (r) => r.timings.duration < 200,
  });
  sleep(1);
}

Troubleshooting

Issue	Cause	Solution
Flaky tests	Race conditions	Use mocks, fix async
Slow suite	Too many E2E	Optimize pyramid
Low coverage	Untested edges	Add boundary tests

Examples

Unit Test (pytest)

Integration Test

Load Test (k6)

Troubleshooting

Resources

testing

Testing Skill

Quick Start

Testing Pyramid