Type checkers

A type checker reads annotations, infers types where there are none, and reports any place a value can’t have the type the code needs. TypeScript’s tsc does this for JavaScript. In Python, PEP 484 (2014) standardised the annotation syntax but deliberately left checking to separate tools; the interpreter stores annotations and ignores them.

Python therefore has several checkers that agree on the typing specification and differ where it is silent. mypy, started by Jukka Lehtosalo in 2012, was the reference implementation. Microsoft’s Pyright, written in TypeScript, evaluates types lazily so it can power an editor and is often 3 to 5 times faster than mypy on large codebases; basedpyright is a stricter community fork. Two Rust checkers arrived in 2025: Meta’s Pyrefly and Astral’s ty.

Each lets you pick how strict to be (Pyright’s basic, standard and strict modes; Pyrefly’s presets from basic to strict), and each runs as a language server, so the errors CI reports appear in the editor as you type.