Reference library

Testing & modern typing

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

7 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 easy

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…

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 easy

How to Compare Floats in pytest with approx

Uses pytest.approx to compare floating-point numbers with tolerance, avoiding precision issues.

pytest floating-point testing

Python

import pytest

def test_float_addition():
    result = 0.1 + 0.2
    expected = 0.3
    assert result == pytest.approx(expected)

13 0 Open

Testing & modern typing easy

How to Use mock.assert_called_with in Python

Verify that a MagicMock received a call with specific positional and keyword arguments using assert_called_with in unittest.

unittest mock testing

Python

import unittest
from unittest.mock import MagicMock

class TestMockAssertions(unittest.TestCase):
    def test_assert_called_with(self):
        # Create a mock object
        mock = MagicMock()

        # Call the mock with specific arguments
        mock.send_email("alice@example.com", subject="Greetings", body="Hel…

14 0 Open

Testing & modern typing easy

How to Write a pytest Test Function with assert Equal in Python

Define simple pytest test functions that use assert to verify result equality and run them with pytest.main.

pytest unit testing assert

Python

import pytest

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

def test_add_positive_numbers():
    result = add(2, 3)
    assert result == 5

def test_add_negative_numbers():
    result = add(-2, -3)
    assert result == -5

def test_add_mixed_numbers():
    result = add(2, -3)
    assert result == -1

if __name__ == "__main__":
  …

11 0 Open

Testing & modern typing easy

How to Write pytest Test Function Assert Equal in Python

Write three pytest test functions that assert the result of an add() function equals an expected numeric value.

pytest assert testing

Python

import pytest

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

def test_add_positive_numbers():
    assert add(2, 3) == 5

def test_add_negative_numbers():
    assert add(-1, -2) == -3

def test_add_mixed_numbers():
    assert add(5, -3) == 2

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

12 0 Open

Browse by section

Each section groups closely related Python snippets.