mypy

Mypy is an open-source static type checker for Python. It analyzes Python programs that include optional type hints and annotations to detect type errors before runtime. Built around the concept of gradual typing, mypy lets developers add type information to existing codebases incrementally while still allowing untyped code to run.

Mypy relies on PEP 484 style type hints from the typing module and related tooling. It examines function signatures, class definitions, and variable annotations, and can also use stub files (.pyi) to describe the interfaces of libraries that lack annotations. It checks type relations and supports type inference in many cases. The tool is run separately from the Python interpreter and can be configured to enforce varying levels of strictness.

Configuration and usage: mypy operates through a command line interface and can be invoked on single modules or entire projects. It supports incremental mode to reuse previous results and caches analysis in a dedicated directory. Projects can tailor behavior with configuration files and options, including enabling strict optional checks and integrating with framework-specific plugins or editor tooling.

Limitations and scope: static type checking cannot fully guarantee correctness for all dynamic Python features, such as runtime code generation, reflection, or complex metaprogramming. Without type hints or accurate stubs for libraries, some errors may go undetected. Nevertheless, mypy is widely used to improve code quality, documentation, and maintainability by making types explicit and enabling early error detection.

Development and ecosystem: mypy is part of the Python typing ecosystem and is maintained as an open-source project. It is commonly used in development workflows and continuous integration to catch type-related issues as code evolves.