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Testing & modern typing

pytest basics, mocks, type hints, TypedDict, Protocol, and static-checking patterns.

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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.

mock unittest file-io

Python

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):…

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Testing & modern typing easy

How to Mock requests.get in Python

Mock requests.get with unittest.mock to test code that makes HTTP calls without hitting the network.

mocking requests unit-testing

Python

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…

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Testing & modern typing easy

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.

unittest mock magicmock

Python

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…

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Testing & modern typing easy

How to Mock subprocess.run returncode in Python

Simulate subprocess.run return codes in tests with unittest.mock.patch and CompletedProcess.

unittest mock subprocess

Python

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…

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Testing & modern typing easy

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.

contract-testing unittest mock

Python

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…

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Testing & modern typing easy

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.

datetime mock unittest

Python

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…

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