Testing in Python encompasses a rich ecosystem of frameworks, tools, and methodologies for verifying code correctness through unit tests, integration tests, and functional tests. The landscape is dominated by pytest (the modern standard with powerful fixtures and plugins) and unittest (the built-in framework following xUnit patterns), though both share the fundamental goal of catching bugs before production. The key insight: testing isn't just about running assertions—it's about structuring tests for maintainability, isolation, and speed, using fixtures to manage setup/teardown, mocks to control dependencies, and parametrization to test multiple scenarios without code duplication.