Testing & modern typing
pytest basics, mocks, type hints, TypedDict, Protocol, and static-checking patterns.
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 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.
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…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 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.
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"])How to Parse Data with Type Hints in Python
A beginner-friendly helper that parses simple dictionary- or list-like strings into typed Python structures using modern typing annotations.
from typing import Any, Dict, List, Union
def parse_data(raw: str) -> Union[Dict[str, Any], List[Any], str]:
"""Parse a simple string into structured data using type hints."""
cleaned = raw.strip()
if not cleaned:
return {}
if cleaned.startswith("{") and cleaned.endswith("}"):
…How to Run Test Coverage with pytest-cov in Python
Run pytest with coverage reporting using pytest-cov on a temporary project and see line-by-line coverage output.
import os
import subprocess
import tempfile
from pathlib import Path
def sample_function(x: int) -> int:
"""A simple function to demonstrate coverage."""
if x > 0:
return x * 2
else:
return -x
def run_pytest_with_coverage() -> str:
"""Run pytest with coverage on a temp project and r…How to Run an Integration Test with Docker Compose Mock in Python
Run a Python integration test against a docker-compose environment, using mocks to simulate service health and business logic responses.
import subprocess
import json
from typing import Dict
def run_integration_test() -> Dict[str, str]:
"""
Simulates an integration test against a docker-compose environment
using a mock service that returns canned responses.
"""
# Mock docker-compose environment check
env_ready = subprocess.run(…How to Share Fixtures Across Tests with pytest conftest
Learn how to define pytest fixtures in conftest.py and control their scope (function, module, session) so every test in a directory reuses the same setup and teardown.
import pytest
@pytest.fixture
def sample_data():
"""Simple fixture available to all tests in this directory."""
return {"name": "Alice", "age": 30}
@pytest.fixture(scope="session")
def session_data():
"""Fixture created once per test session."""
return {"session_id": 12345}
@pytest.fixture(scope="mo…How to Skip Slow Tests with pytest.mark in Python
Use pytest.mark.skip and custom marks like @pytest.mark.slow to skip or deselect slow tests during test runs.
import pytest
import time
def test_fast():
assert 1 + 1 == 2
@pytest.mark.skip(reason="slow test skipped by default")
def test_slow():
time.sleep(5)
assert True
@pytest.mark.slow
def test_marked_slow():
time.sleep(5)
assert True
if __name__ == "__main__":
pytest.main([__file__, "-v", "-…How to Snapshot Test JSON with Mock in Python
Use pytest-snapshot to capture the exact output of a JSON-loading function, with and without mocking json.loads, so future changes are automatically detected.
import json
from unittest.mock import Mock, patch
import pytest
def load_config(data):
config = json.loads(data)
return {"host": config["host"], "port": config["port"]}
def test_load_config_snapshot(snapshot):
mock_data = json.dumps({"host": "localhost", "port": 8080, "extra": "ignored"})
result = …How to Sort Data in Python
Sort sequences with type-safe helpers that handle mixed data with a string fallback.
from typing import Any, TypeVar, Protocol, Sequence, Iterable
T = TypeVar("T")
Comparable = TypeVar("Comparable", bound="Comparable")
class Sortable(Protocol):
def __lt__(self, other: Any) -> bool: ...
S = TypeVar("S", bound=Sortable)
def sort_data(data: Sequence[S], *, reverse: bool = False) -> list[S]:
"…How to Test Environment Variables with pytest monkeypatch in Python
Shows how to use pytest's monkeypatch fixture to set and delete environment variables for isolated tests.
import os
import pytest
def get_database_url():
return os.getenv("DATABASE_URL", "postgres://default")
def test_database_url_with_env(monkeypatch):
monkeypatch.setenv("DATABASE_URL", "postgres://test-db")
assert get_database_url() == "postgres://test-db"
def test_database_url_default(monkeypatch):
m…How to Test Hypotheses with Property-Based Check in Python
A Python search that checks an integer property (palindrome divisible by digit sum) and returns the first counterexample within a range, with exactly reproduced output from the code.
def is_property_satisfied(n):
"""
Demonstrates a mathematically inspired property:
checks whether n is both a palindrome and divisible by its digit sum.
"""
s = str(n)
if s != s[::-1]:
return False
digit_sum = sum(int(d) for d in s)
return digit_sum != 0 and n % digit_sum == 0
…How to Test Properties with Random Inputs in Python
Write a simple property-based test in Python using random string generation to verify that string invariants like reverse-twice identity and uppercase idempotence always hold.
import random
import string
def generate_random_string(length: int) -> str:
"""Generate a random alphanumeric string of given length."""
chars = string.ascii_letters + string.digits
return "".join(random.choice(chars) for _ in range(length))
def reverse_twice_is_identity(s: str) -> bool:
"""Propert…How to Use Basic Type Hints (int, str) for Return Values in Python
Declare a simple function with int and str type hints and a typed return value in Python.
def greet(name: str, age: int) -> str:
return f"{name} is {age} years old."
if __name__ == "__main__":
print(greet("Alice", 30))How to Use Hypothesis Strategies for Lists of Text in Python
Generate random lists of non-empty strings with Hypothesis and verify that joining them with a comma-and-space separator meets expected length and containment invariants.
from hypothesis import given, strategies as st
from hypothesis import example
@given(st.lists(st.text(min_size=1, max_size=10), min_size=1, max_size=5))
def test_joined_string_length(items):
"""Each text is non-empty; a joined string should be at least as long
as the number of items (separator adds character…How to Use Literal Type Hints in Python
Use typing.Literal to restrict a function parameter to specific allowed string values and get static type checking.
from typing import Literal
def get_status_message(status: Literal["active", "inactive", "pending"]) -> str:
"""Return a message based on the status value."""
if status == "active":
return "Account is active"
elif status == "inactive":
return "Account is inactive"
else:
return "…How to Use Mock Flip Mutation Testing in Python
Demonstrates how mutation testing tools flip Boolean literals (mock flip) in Python source to verify test suite effectiveness in catching logic changes.
import random
# In mutation testing, a "mock flip" intentionally changes a Boolean
# constant to False (or True) to see if the test suite catches it.
# This is a common "constant mutation" applied to a source file's literals.
def is_even(n: int) -> bool:
"""Return True if n is even. Contains a Boolean literal us…How to Use Python Type Hints for Beginners
Build a data helper module with basic type hints — Union, Optional, List, Dict, Any, and TypeVar — to make your code clearer and safer.
from typing import Any, Union, Optional, List, Dict, Tuple, Callable, TypeVar
T = TypeVar("T")
def describe(value: Any) -> str:
"""Return a human-readable description of the value's type."""
if isinstance(value, list):
return f"list of {len(value)} items"
elif isinstance(value, dict):
ret…How to Use Stubs, Fakes, Spies, and Mocks in Python Testing
Implement four types of test doubles — stubs, fakes, spies, and mocks — as subclasses of a PaymentGateway interface to replace real dependencies during testing.
class PaymentGateway:
def charge(self, amount):
raise NotImplementedError
class StubPaymentGateway(PaymentGateway):
"""Returns a fixed response without any logic."""
def charge(self, amount):
return {"success": True, "transaction_id": "stub-12345"}
class FakePaymentGateway(PaymentGatewa…How to Use TypedDict and Dataclasses in Python
Create typed data structures with TypedDict and dataclasses, then use them as helper functions for describing objects in a type-safe way.
from typing import TypedDict, NotRequired, Optional
from dataclasses import dataclass
class User(TypedDict):
name: str
age: NotRequired[int]
email: Optional[str]
@dataclass
class Product:
id: int
title: str
price: float = 0.0
def describe_user(user: User) -> str:
age = user.get("age",…How to Use TypedDict for Data Validation in Python
Define a TypedDict schema and validate raw dictionary input with type hints for safer, more readable data handling.
from typing import Any, Dict, List, Optional, Union, TypedDict, Literal
class Product(TypedDict):
product_id: int
name: str
price: Union[int, float]
in_stock: bool
tags: Optional[List[str]]
def validate_product(data: Dict[str, Any]) -> Product:
product_id: int = int(data["product_id"])
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