Testing & modern typing
pytest basics, mocks, type hints, TypedDict, Protocol, and static-checking patterns.
Format Data with Type Hints in Python
Build a validated person dict with modern type hints and optional list handling.
from typing import Any, Dict, List, Optional, Union
JsonValue = Union[str, int, float, bool, None, List["JsonValue"], Dict[str, "JsonValue"]]
def format_person(name: str, age: int, hobbies: Optional[List[str]] = None) -> Dict[str, Any]:
"""Build a person dict with validated typing."""
if not name or age < 0:…How to Capture Logging Records with pytest caplog in Python
Capture and assert on logging records in pytest using the built-in caplog fixture.
import logging
import pytest
def divide(a, b):
"""Divide two numbers and log an error if b is zero."""
if b == 0:
logging.error("Division by zero attempted")
return None
logging.info(f"Dividing {a} by {b}")
return a / b
def test_divide_logs_error(caplog):
with caplog.at_level(logg…How to Merge TypedDicts in Python
Merge two TypedDict dictionaries with type-aware logic using NotRequired, **kwargs unpacking, and safe key updates.
from typing import TypedDict, NotRequired, merge # hypothetical
class User(TypedDict):
name: str
email: NotRequired[str]
age: NotRequired[int]
def merge_users(base: User, **overrides: User) -> User:
"""Merge two user dicts with typing-aware logic."""
result: User = dict(base)
for key, value …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 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 the pytest tmp_path Fixture for Temporary Directories
Use pytest's built-in tmp_path fixture to create a unique temporary directory per test for clean file I/O testing.
import pytest
def test_write_and_read_file(tmp_path):
# tmp_path is a pytest fixture that provides a temporary directory
# unique to each test invocation
data_file = tmp_path / "data.txt"
data_file.write_text("hello world")
assert data_file.read_text() == "hello world"
def test_multiple_tmp_pat…How to Validate Data in Python with Typing Hints
Build a runtime validation helper that checks values against Python type hints like Optional, list, and basic types.
from typing import Any, Optional, Union, TypeVar, get_origin, get_args
T = TypeVar("T")
def validate(value: Any, expected_type: type) -> Optional[str]:
"""Returns an error message if value doesn't match expected_type, else None."""
# Handle Optional[...] types
origin = get_origin(expected_type)
if or…How to Write a Fast Smoke Test for a Critical Path in Python
A quick smoke test that validates the /health critical path executes fast enough, raising errors on wrong paths or slow responses.
import time
def smoke_test(path):
if path != "/health":
raise ValueError("Critical path expected /health")
start = time.perf_counter()
# Simulate the critical health check work
time.sleep(0.01)
elapsed = time.perf_counter() - start
if elapsed > 0.05:
raise RuntimeError("Health …Browse by section
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