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Testing & modern typing

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

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How to Assert Exceptions in Python with pytest.raises

Use pytest.raises as a context manager to assert that a function raises an expected exception and inspect its message in pytest tests.

pytest testing exceptions

Python

import pytest

def divide(a, b):
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

def test_divide_by_zero():
    with pytest.raises(ValueError) as exc_info:
        divide(10, 0)
    assert str(exc_info.value) == "Cannot divide by zero"
    assert "zero" in str(exc_info.value)

def te…

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Testing & modern typing easy

How to Compare Files and Show a Diff in Python

Compare two text files and print a unified diff using Python's difflib module to highlight differences.

diff files difflib

Python

import difflib
from pathlib import Path

def compare_files(expected_path: str, actual_path: str) -> str:
    """Compare two text files and return a unified diff."""
    expected = Path(expected_path).read_text()
    actual = Path(actual_path).read_text()

    diff = difflib.unified_diff(
        expected.splitlines(ke…

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Testing & modern typing medium

How to Mock and Stub API Calls in Playwright E2E Tests with Python

This code demonstrates how to mock and stub API responses in Playwright end-to-end tests using Python's unittest.mock patch and Playwright's APIRequestContext.

playwright e2e-testing mocking

Python

import re
from unittest.mock import patch
from playwright.sync_api import sync_playwright

def verify_api_mock(page, mock_url, mock_response):
    with patch("playwright.sync_api.APIRequestContext.get") as mock_get:
        mock_get.return_value.json.return_value = mock_response
        mock_get.return_value.status_co…

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Testing & modern typing easy

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.

testing mock pathlib

Python

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…

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Testing & modern typing medium

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.

hypothesis property-based-testing strategies

Python

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…

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