mypy

Start with one annotated function instead of trying to type an entire Python project at once. The getting-started guide explains how annotations and the typing module give mypy information to check. For example, a function that accepts a list of integers and returns an integer can declare those types; mypy can then report a call that passes an incompatible value. A key choice is how much of the codebase is annotated. The guide states that unannotated functions are treated as dynamically typed by default, so they receive less checking unless stricter options are enabled. This allows gradual adoption, but it also means a clean result does not establish that all Python code has been checked. Use the command-line guide and configuration options to decide which modules and strictness level fit the project. mypy checks types before execution; it does not change Python's runtime behavior or validate external JSON by itself. The project is MIT-licensed, and its documentation describes mypy and mypyc as related components. For style and formatting checks, Ruff is a separate tool with a different purpose.