A static type checker for Python that uses type hints (PEP 484) to verify type correctness without running code — catches whole classes of bugs at lint time.
Mypy analyzes Python code annotated with type hints and reports type errors: passing wrong argument types, accessing attributes that don't exist, mixing Optional with non-Optional, etc. It supports strict mode for new code and gradual typing for legacy codebases. Integrates with IDEs (via the Language Server Protocol) for inline type errors and supports plugins for Django, Pydantic, and other type-heavy libraries.
Adding `mypy --strict` to CI on a new microservice to catch every type mismatch before merge — eliminates entire categories of runtime errors.
Static type checking shifts bug detection left — finding errors in seconds locally instead of hours later in production logs.
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