pytest

Warnings

• breaking pytest.warns(None) removed in 8.0. It was used to assert no warnings were emitted, but its semantics were inverted — it actually asserted at least one warning was raised. This pattern is very common in LLM-generated test code.
  Fix: To assert no warnings: use warnings.catch_warnings() or recwarn fixture and assert len(recwarn) == 0. To assert a specific warning: pytest.warns(UserWarning).
• breaking @pytest.yield_fixture removed. LLMs frequently generate this decorator as it appeared in tutorials for years.
  Fix: Replace @pytest.yield_fixture with @pytest.fixture. yield inside @pytest.fixture has been supported since pytest 3.0.
• breaking nose plugin support removed in 8.0. Tests written for nose (using setup/teardown at module level, nose-style generators) will fail to collect.
  Fix: Rewrite nose-style tests using pytest fixtures and @pytest.mark.parametrize.
• breaking Python 3.9 support dropped in pytest 9.0 (following Python 3.9 EOL). Tests on Python 3.9 must pin to pytest<9.
  Fix: Pin pytest<9 for Python 3.9 environments.
• breaking File/directory collection order changed in pytest 8.x. Files and directories are now collected alphabetically together. Previously, files were collected before directories. Test suites with ordering assumptions may break.
  Fix: Do not rely on implicit test collection order. Use pytest-ordering if explicit order is needed.
• gotcha Async tests require pytest-asyncio. pytest does not natively run async def test_ functions — they will appear to pass silently without actually running the test body.
  Fix: pip install pytest-asyncio. Add asyncio_mode = 'auto' to pytest.ini or mark individual tests with @pytest.mark.asyncio.
• gotcha --strict renamed to --strict-markers (for marker strictness) and --strict-config (for config strictness). The bare --strict still works in 9.x but now enables full strict mode.
  Fix: Use --strict-markers to fail on undeclared markers. Declare all markers in pytest.ini under [pytest] markers.
• gotcha pytest_plugins defined in non-root conftest.py files is deprecated. It activates plugins globally, not just for tests below that conftest.
  Fix: Define pytest_plugins only in the root conftest.py or pyproject.toml.

Install

• pip install pytest Python
• pip install pytest pytest-cov pytest-asyncio Common test stack

Imports

• pytest

  import pytest

  Always import as 'import pytest'. The py.test command name is legacy; use the pytest command.
• pytest.fixture

  @pytest.fixture
  def my_fixture(): ...

  @pytest.yield_fixture was an alias for @pytest.fixture. It was deprecated in 7.x and removed. Use @pytest.fixture with yield directly.

Quickstart

Basic fixture and parametrize usage. Run with: pytest -v

# test_example.py
import pytest

@pytest.fixture
def sample_data():
    return {"key": "value"}

def test_basic(sample_data):
    assert sample_data["key"] == "value"

@pytest.mark.parametrize("x,expected", [(1, 2), (2, 4)])
def test_double(x, expected):
    assert x * 2 == expected

# Run: pytest -v