Testing & modern typing
pytest basics, mocks, type hints, TypedDict, Protocol, and static-checking patterns.
How to Mock and Stub API Calls in Playwright E2E Tests with Python
This code demonstrates how to mock and stub API responses in Playwright end-to-end tests using Python's unittest.mock patch and Playwright's APIRequestContext.
import re
from unittest.mock import patch
from playwright.sync_api import sync_playwright
def verify_api_mock(page, mock_url, mock_response):
with patch("playwright.sync_api.APIRequestContext.get") as mock_get:
mock_get.return_value.json.return_value = mock_response
mock_get.return_value.status_co…How to Mock open() in Python for Reading File Data
This example shows how to mock Python's built-in open() function using unittest.mock to simulate file reading without touching the disk.
import builtins
from unittest.mock import patch
def read_file_data(filename):
with open(filename, 'r') as f:
return f.read()
def mock_read_data():
fake_data = "This is mocked file content"
class FakeFile:
def __enter__(self):
return self
def __exit__(self, *args):…How to Mock requests.get in Python
Mock requests.get with unittest.mock to test code that makes HTTP calls without hitting the network.
import requests
from unittest.mock import Mock, patch
def fetch_user_data(user_id):
response = requests.get(f"https://api.example.com/users/{user_id}")
return response.json()
def process_user(user_id):
mock_response = Mock()
mock_response.json.return_value = {"id": user_id, "name": "Alice", "age": 30…How to Mock return_value with MagicMock in Python unittest
Use unittest.mock.MagicMock to replace a dependency and set return_value to control what a mocked method returns during unit tests.
import unittest
from unittest.mock import MagicMock
class PaymentGateway:
def charge(self, amount):
raise NotImplementedError
class OrderService:
def __init__(self, gateway):
self.gateway = gateway
def process_order(self, amount):
return self.gateway.charge(amount)
class Test…How to Mock subprocess.run returncode in Python
Simulate subprocess.run return codes in tests with unittest.mock.patch and CompletedProcess.
import subprocess
from unittest.mock import patch
def run_command(cmd):
result = subprocess.run(cmd, capture_output=True, text=True)
return result.returncode
if __name__ == "__main__":
with patch("subprocess.run") as mock_run:
# Simulate a successful command (returncode 0)
mock_run.retu…How to Write a Contract Test with Mock in Python
Use unittest.mock to verify a consumer's expectations match the provider's response shape in a Python contract test.
from unittest.mock import Mock
# Contract test: verify consumer expects data shape that provider delivers.
# We mock the provider and assert the consumer's calls match the agreed contract.
def fetch_user(provider_client, user_id):
"""Consumer code: expects provider to return {'id', 'name', 'email'}."""
respo…Mock datetime.now to freeze time in Python
Use unittest.mock.patch to replace datetime.now with a fixed value so your code always sees the same time during tests.
from datetime import datetime
from unittest.mock import patch
def current_message():
now = datetime.now()
return f"Current time: {now:%Y-%m-%d %H:%M:%S}"
if __name__ == "__main__":
with patch("__main__.datetime") as mock_dt:
mock_dt.now.return_value = datetime(2024, 3, 15, 10, 30, 0)
prin…Use pytest fixture to mock a database connection in Python
This code shows how to use a pytest fixture and unittest.mock to replace a database connection with a Mock, enabling isolated tests without a real database.
import pytest
import sqlite3
from unittest.mock import Mock
class Database:
def __init__(self, connection):
self.connection = connection
def get_user(self, user_id):
cursor = self.connection.cursor()
cursor.execute("SELECT * FROM users WHERE id = ?", (user_id,))
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