Pytest

If you're writing Python tests, this is the framework you should be using. It handles everything from simple assertions to complex integration tests with fixtures for dependency injection, parametrization for data-driven testing, and plugins for coverage, async, and mocking. The fixture system is genuinely better than setup/teardown methods, and parametrize decorators let you run the same test with different inputs without copy-pasting code. Works seamlessly with FastAPI, Django, and Flask, supports async/await natively, and the assertion introspection means you just write assert statements instead of assertEqual noise. Industry standard for good reason.