Implementing API Mocking for Automation Testing

Pratik Chavan
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Article

API mocking is a technique used to simulate the behavior of real APIs during testing. It helps isolate your system under test, test edge cases, and avoid dependencies on external services.

Tools for API Mocking

Choose a tool based on your tech stack:

Java: WireMock, MockServer
JavaScript: MSW (Mock Service Worker), Nock
Python: responses, httpretty
Postman: Postman Mock Server
Standalone: Beeceptor, Mocky

Setup Steps

1. Define Mock Endpoints

Specify

URL (e.g., /api/users)
HTTP method (GET, POST, etc.)
Response body (JSON/XML)
Status code (200, 404, etc.)
Headers (optional)

Example (WireMock JSON config)

{
  "request": {
    "method": "GET",
    "url": "/api/users/123"
  },
  "response": {
    "status": 200,
    "body": "{\"id\":123,\"name\":\"Pratik\"}",
    "headers": {
      "Content-Type": "application/json"
    }
  }
}

2. Integrate with Your Test Framework

JavaScript (MSW Example)

import { setupServer } from 'msw/node';
import { rest } from 'msw';

const server = setupServer(
  rest.get('/api/users/:id', (req, res, ctx) => {
    return res(ctx.status(200), ctx.json({ id: req.params.id, name: 'Pratik' }));
  })
);

beforeAll(() => server.listen());
afterEach(() => server.resetHandlers());
afterAll(() => server.close());

Python (responses Example)

import responses
import requests

@responses.activate
def test_api_mock():
    responses.add(
        responses.GET,
        'http://example.com/api/users/123',
        json={'id': 123, 'name': 'Pratik'},
        status=200
    )
    resp = requests.get('http://example.com/api/users/123')
    assert resp.json() == {'id': 123, 'name': 'Pratik'}

Best Practices

Version control your mocks: Store mock definitions in Git.
Use environment flags: Switch between real and mock APIs.
Simulate failures: Test timeouts, 500 errors, and invalid responses.
Keep mocks realistic: Use tools like Faker for dynamic data.
Automate mock server startup: Integrate with CI/CD pipelines.