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Testing & modern typing

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

3 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…

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Testing & modern typing medium

How to Flag Unexpected Diff Changes in Python

Compares two snapshot lists, detects unexpected differences, and returns a flag indicating whether the snapshot should be updated.

diffing snapshot-testing difflib

Python

import difflib

def snapshot_diff(before, after, intentional_changes=None):
    """Compare snapshots and flag only unexpected differences."""
    intentional_changes = intentional_changes or set()
    diff = list(difflib.unified_diff(before, after, lineterm=""))
    has_unexpected = False

    for line in diff:
      …

16 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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