pytest is Python's leading testing framework, built on the principle that tests should be simple to write and powerful to execute. Unlike unittest's verbose class-based approach, pytest uses plain assert statements with automatic introspection that shows you exactly what failed and why, fixture dependency injection for clean test setup, and a rich plugin ecosystem with 800+ extensions. The framework's key innovation is making the common case trivial—write def test_something(): assert result == expected—while providing advanced features like parametrization, markers, and parallel execution when complexity demands them.