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Testing & modern typing

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

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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 easy

How to Parametrize pytest Tests with Multiple Input Cases in Python

This code shows how to use pytest's @pytest.mark.parametrize decorator to run the same test function across multiple input-output combinations, checking that an add function behaves correctly for each case.

pytest parametrize testing

Python

import pytest

def add(a, b):
    return a + b


@pytest.mark.parametrize("a,b,expected", [
    (1, 2, 3),
    (5, 5, 10),
    (-1, 1, 0),
    (0, 0, 0),
    (10, -3, 7),
])
def test_add(a, b, expected):
    assert add(a, b) == expected


if __name__ == "__main__":
    pytest.main([__file__, "-v"])

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Testing & modern typing easy

How to Test Hypotheses with Property-Based Check in Python

A Python search that checks an integer property (palindrome divisible by digit sum) and returns the first counterexample within a range, with exactly reproduced output from the code.

hypothesis testing palindrome

Python

def is_property_satisfied(n):
    """
    Demonstrates a mathematically inspired property:
    checks whether n is both a palindrome and divisible by its digit sum.
    """
    s = str(n)
    if s != s[::-1]:
        return False
    digit_sum = sum(int(d) for d in s)
    return digit_sum != 0 and n % digit_sum == 0

…

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Testing & modern typing easy

How to Verify Formatted Output with an Approval Test in Python

Write a small Python approval test that verifies a function's exact formatted output using unittest.

approval-testing unittest formatting

Python

import sys
from io import StringIO
import unittest

def generate_output(name, score):
    return f"Player: {name} | Score: {score:03d}"

class TestFormattedOutput(unittest.TestCase):
    def test_output_format(self):
        expected = "Player: Alice | Score: 042"
        result = generate_output("Alice", 42)
        …

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