Mypy

Last updated: Oct 28, 2026

Rationale

Fluid Attacks uses Mypy as the primary static type checker for Python source code.

It is one of the first Python type checkers. Thus, it has many implemented rules, and it is carefully tested.
It is open source.
It has a very large number of typing checks.
It supports caching of reviewed files, for performance.
It does not have a language server by default, which makes the development experience much worse.
It integrates in-editor typing with its VS Code extension for the development experience.
Because it is built in Python, it has serious performance limitations.

At this time, Fluid Attacks examines these alternatives as possible replacements for Mypy:

Ty is a static type checker that Astral develops. Astral also made uv and Ruff. Ty is in development at this time.
It is open source.
It does not have as many typing checks as Mypy.
It does not support caching at this time, but it is much faster than Mypy.
It has a language server by default, which makes the development experience much better.
It integrates in-editor typing with its VS Code extension for the development experience.
Because it is built in Rust, it is much faster than other type checkers. Thus, performance is not a problem with Ty.

Fluid Attacks last reviewed Ty on Aug 1, 2025.

Pyright is a static type checker that Microsoft develops. After Mypy, it is the most used type checker, and it is production-ready.
It is open source.
It has approximately the same number of typing checks as Mypy, or possibly more.
It does not support caching at this time, but it is much faster than Mypy.
It has a language server by default, which makes the development experience much better.
It integrates in-editor typing with its VS Code extension for the development experience.
Because it is built in TypeScript, it is faster than Mypy, but slower than the Rust-based type checkers.

Fluid Attacks last reviewed Pyright on Aug 1, 2025.

Pyrefly is a static type checker that Meta develops. It is in development at this time.
It is open source.
It does not have as many typing checks as Mypy.
It does not support caching at this time, but it is much faster than Mypy.
It has a language server by default, which makes the development experience much better.
It integrates in-editor typing with its VS Code extension for the development experience.
Because it is built in Rust, it is much faster than other type checkers. Thus, performance is not a problem with Pyrefly.

Fluid Attacks last reviewed Pyrefly on Aug 1, 2025.

Fluid Attacks uses Mypy to enforce strict static type checking on all its Python source code.