Mypy

Rationale

We use Mypy as the main static type checker for Python source code. Mypy:

Is an old-school Python type checker, which means it has many rules implemented and has been thoroughly tested
Is open source
Has a huge number of typing checks
Supports caching already-reviewed files for performance
Does not provide a language server out of the box, considerably affecting development experience
Integrates in-editor typing using its VS Code extension for development experience
As it is built in Python, it has serious performance limitations

Below are alternatives we are currently considering as possible replacements to Mypy.

Ty is a static type checker developed by Astral, creators of uv and Ruff; it is still under development
Is open source
Does not have as many typing checks as Mypy
Although it currently does not support caching, it is still much faster compared to Mypy
Provides a language server out of the box, considerably improving development experience
Integrates in-editor typing using its VS Code extension for development experience
As it is built in Rust, it is considerably faster compared to other type checkers, up to a point where performance is no longer a concern

Ty was last reviewed on Aug 1, 2025.

Pyright is a static type checker developed by Microsoft; after Mypy, it is the most widely-used type checker and is production-ready
Is open source
Is very close to Mypy, maybe even ahead, in terms of typing checks
Although it currently does not support caching, it is still much faster compared to Mypy
Provides a language server out of the box, considerably improving development experience
Integrates in-editor typing using its VS Code extension for development experience
As it is built in TypeScript, it is faster than Mypy, but still slower than Rust-based ones

Pyright was last reviewed on Aug 1, 2025.

Pyrefly is a static type checker developed by Meta; it is still under development
Is open source
Does not have as many typing checks as Mypy
Although it currently does not support caching, it is still much faster compared to Mypy
Provides a language server out of the box, considerably improving development experience
Integrates in-editor typing using its VS Code extension for development experience
As it is built in Rust, it is considerably faster compared to other type checkers, up to a point where performance is no longer a concern

Pyrefly was last reviewed on Aug 1, 2025.

We use Mypy to enforce strict static type checking on all of Fluid Attacks' Python source code.