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Testing & modern typing

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

4 matches

Capture stdout and stderr with pytest capsys

Use pytest's capsys fixture to capture and assert on standard output and error streams in your tests.

import pytest

# Function under test
def greet(name):
    print(f"Hello, {name}!")
    print(f"Error: {name} not found", file=sys.stderr)

def test_captures_stdout_and_stderr(capsys):
    greet("Alice")
    captured = capsys.readouterr()
    
    assert "Hello, Alice!" in captured.out
    assert "Error: Alice not foun…

12 0 Open

Testing & modern typing medium

Characterization Test for Legacy Python Code

Capture the exact output of a legacy Python function for known inputs, creating a characterization test that documents current behavior before refactoring.

characterization-testing legacy-code testing

Python

def legacy_behavior(value):
    """Legacy function that returns a tuple with unconventional types."""
    if value == "special":
        return None, "legacy-special"
    elif value > 100:
        return value, "large"
    elif value > 0:
        return value * 2, "positive-doubled"
    elif value == 0:
     …

14 0 Open

Testing & modern typing easy

How to Capture Logging Records with pytest caplog in Python

Capture and assert on logging records in pytest using the built-in caplog fixture.

pytest logging testing

Python

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…

15 0 Open

Testing & modern typing medium

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.

pytest snapshot mock

Python

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 = …

14 0 Open

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