I built a pytest plugin that enforces test size categories (small, medium, large, xlarge) based on Google's "Software Engineering at Google" testing philosophy.

The problem: flaky tests. They erode trust, waste time, and make CI a nightmare.

The solution: enforce hermeticity by category. Small tests can't access the network, filesystem, or databases. Medium tests are limited to localhost. Each size has time limits (1s, 5min, 15min).

When a test violates its constraints, you get an actionable error:

       [TC001] Network Access Violation
    Test: test_api.py::test_fetch_user [SMALL]
    
    What happened:
        Attempted network connection to api.example.com:443

    To fix this:
      • Mock with responses/httpretty/respx
      • Use dependency injection
      • Change to @pytest.mark.medium

Mocking libraries (responses, pyfakefs) still work - they intercept before the blocking layer.

The goal is maintaining a healthy test pyramid: 80% small, 15% medium, 5% large.

GitHub: https://github.com/mikelane/pytest-test-categories Docs: https://pytest-test-categories.readthedocs.io PyPI: https://pypi.org/project/pytest-test-categories/