Type Checking

The original and canonical static type checker for Python — the PEP 484 reference implementation that defined the modern Python type-hint ecosystem.

Microsoft's fast static type checker for Python — the engine behind Pylance in VS Code, written in TypeScript with strong inference and IDE-first diagnostics.

Astral's new ultra-fast Rust-based Python type checker from the makers of ruff and uv — pre-1.0 but already widely adopted for its speed.

Meta's high-performance Python type checker written in Rust — open-sourced in 2025 as the successor to Pyre, designed for monorepo-scale codebases.