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Testing & modern typing

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

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Fix and Test a Regression Bug in Python with Unit Tests

This code implements a circle area function that raises ValueError for negative radii, then runs basic tests and a regression check for that edge case.

regression-testing unit-testing math

Python

import math

def calculate_area(radius):
    """Calculate the area of a circle given its radius."""
    if radius < 0:
        raise ValueError("Radius cannot be negative")
    return math.pi * radius ** 2

def main():
    test_cases = [0, 1, 2.5, 5, 10]
    
    print("Circle Area Calculator")
    print("-" * 30)
   …

16 0 Open

Testing & modern typing easy

How to Mock pathlib Path.read_text with mock_open in Python

Mock pathlib.Path.read_text using patch and mock_open to test file-reading code without touching the filesystem.

testing mock pathlib

Python

import pathlib
from unittest.mock import mock_open, patch

def read_config(filepath: pathlib.Path) -> str:
    """Read file content with pathlib."""
    return filepath.read_text()

if __name__ == "__main__":
    mock_data = "version: 1.0\nname: demo-app"
    
    with patch("pathlib.Path.open", mock_open(read_data=mo…

14 0 Open

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…

12 0 Open

Testing & modern typing easy

How to Parametrize pytest Tests with Multiple Input Cases in Python

This code shows how to use pytest's @pytest.mark.parametrize decorator to run the same test function across multiple input-output combinations, checking that an add function behaves correctly for each case.

pytest parametrize testing

Python

import pytest

def add(a, b):
    return a + b


@pytest.mark.parametrize("a,b,expected", [
    (1, 2, 3),
    (5, 5, 10),
    (-1, 1, 0),
    (0, 0, 0),
    (10, -3, 7),
])
def test_add(a, b, expected):
    assert add(a, b) == expected


if __name__ == "__main__":
    pytest.main([__file__, "-v"])

14 0 Open

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…

13 0 Open

Testing & modern typing easy

How to Write pytest Test Function Assert Equal in Python

Write three pytest test functions that assert the result of an add() function equals an expected numeric value.

pytest assert testing

Python

import pytest

def add(a, b):
    return a + b

def test_add_positive_numbers():
    assert add(2, 3) == 5

def test_add_negative_numbers():
    assert add(-1, -2) == -3

def test_add_mixed_numbers():
    assert add(5, -3) == 2

if __name__ == "__main__":
    pytest.main([__file__, "-v"])

12 0 Open

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