end-to-end (E2E) test

Example

Say a shop charges a customer correctly but its receipt page comes back empty. A test of the pricing function alone can’t catch that, so the test below serves the real app with wsgiref, the standard library’s built-in WSGI server, and walks the purchase over HTTP:

import json
import threading
import unittest
import urllib.request
from wsgiref.simple_server import make_server

from shop import app


def get_json(url, payload=None):
    data = json.dumps(payload).encode() if payload else None
    with urllib.request.urlopen(url, data) as response:
        return json.load(response)


class TestCheckout(unittest.TestCase):
    def setUp(self):
        self.server = make_server("localhost", 0, app)
        threading.Thread(target=self.server.serve_forever, daemon=True).start()
        self.addCleanup(self.server.server_close)
        self.addCleanup(self.server.shutdown)
        self.url = f"http://localhost:{self.server.server_port}"

    def test_customer_can_buy_two_books(self):
        order = get_json(f"{self.url}/checkout", {"PY-101": 2})
        receipt = get_json(f"{self.url}/orders/{order['id']}")

        self.assertEqual(receipt["status"], "paid")
        self.assertEqual(receipt["total"], 59.98)


if __name__ == "__main__":
    unittest.main()

Running the file logs the two requests that crossed the wire before the verdict:

$ python test_checkout_e2e.py
127.0.0.1 - - [07/Aug/2026 02:06:00] "POST /checkout HTTP/1.1" 200 9
127.0.0.1 - - [07/Aug/2026 02:06:00] "GET /orders/1 HTTP/1.1" 200 34
.
----------------------------------------------------------------------
Ran 1 test in 0.006s

OK

The test never calls the checkout code directly. It posts a cart to /checkout, then fetches the resulting order, so the assertion on the receipt total passes only if routing, pricing, and storage work together in the running app.

Plant a fault anywhere in the shop below, then walk the same purchase through it:

Only the broken price shows up in a test of price_cart() on its own. The unmarked order, the missing route, and the unset environment variable never touch that function, so they surface only once a test drives the running app.

Related Resources

Tutorial

Modern Web Automation With Python and Selenium

Learn advanced Python web automation techniques with Selenium, such as headless browsing, interacting with web elements, and implementing the Page Object Model pattern.

intermediate projects testing web-scraping

For additional information on related topics, take a look at the following resources:

• pytest Tutorial: Effective Python Testing (Tutorial)
• Python's unittest: Writing Unit Tests for Your Code (Tutorial)
• Testing Your Code With pytest (Course)
• Testing Your Code With Python's unittest (Course)
• Test-Driven Development With pytest (Course)
• How to Provide Test Fixtures for Django Models in pytest (Tutorial)
• Web Automation With Python and Selenium (Quiz)
• Effective Testing with Pytest (Quiz)
• Python's unittest: Writing Unit Tests for Your Code (Quiz)