Testing & modern typing
pytest basics, mocks, type hints, TypedDict, Protocol, and static-checking patterns.
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.
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:
…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 = …Browse by section
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Testing & modern typing — Python code examples
What you will find here
This page collects testing & modern typing snippets — short, copy-ready Python you can paste into our free online IDE and run without installing anything. Each sample includes a plain-English explanation and the full source code.
Samples vs tutorials and challenges
Samples are quick reference — one concept per page. For step-by-step teaching, use our Python tutorials. To test yourself, try quizzes or coding challenges. Clean up style with the Python formatter.