Asserts

Pytest · Reference cheat sheet

📋 Overview

pytest rewrites assert for rich introspection — no special assert API required. Use pytest.raises, approx, and helper matchers for exceptions and floats.

🔧 Core concepts

Assertion rewriting applies to test modules; helper modules may need pytest.register_assert_rewrite.

💡 Examples

Basics:

def test_list():
    data = [1, 2, 3]
    assert data == [1, 2, 3]
    assert 2 in data
    assert len(data) == 3

Exceptions:

import pytest

def test_raises():
    with pytest.raises(ZeroDivisionError):
        1 / 0

def test_message():
    with pytest.raises(ValueError, match="invalid"):
        raise ValueError("invalid id")

Approx & warnings:

assert 0.1 + 0.2 == pytest.approx(0.3)
assert [0.1, 0.2] == pytest.approx([0.1, 0.2])

with pytest.warns(UserWarning, match="deprecated"):
    warn_user()

Failure messages:

assert result.ok, f"expected ok, got {result!r}"

⚠️ Pitfalls

• Writing assertEqual from unittest without need — plain assert is fine.
• Comparing floats with == without approx.
• Catching exceptions with try/except instead of pytest.raises.
• Huge diffs from comparing large dicts — use focused keys.
• Assert rewriting disabled in some imported helpers.

🔗 Related

• Fixtures
• Parametrize
• Mocking
• capsys & caplog
• CLI
• Coverage