# Pyrefly — Fast Python Type Checker and Language Server by Meta

> A Rust-based Python type checker and language server from Meta that provides fast, accurate type analysis with IDE integration via LSP.

## Install

Save in your project root:

# Pyrefly — Fast Python Type Checker and Language Server by Meta

## Quick Use
```bash
pip install pyrefly
# Check a file
pyrefly check myproject/
# Start language server for IDE integration
pyrefly lsp
```

## Introduction
Pyrefly is a Python type checker and language server developed by Meta. Written in Rust for performance, it analyzes Python code for type errors and provides IDE features like auto-completion, go-to-definition, and hover documentation through the Language Server Protocol.

## What Pyrefly Does
- Type-checks Python code against PEP 484 type annotations at high speed
- Provides a language server with auto-complete, hover, and go-to-definition
- Supports incremental checking for fast feedback during development
- Handles large codebases with millions of lines efficiently
- Integrates with VS Code, Neovim, and other LSP-compatible editors

## Architecture Overview
Pyrefly is implemented in Rust with a multi-phase pipeline: parsing (using a custom Python parser), name resolution, type inference, and error reporting. The language server runs as a persistent process that re-analyzes only changed files and their dependents. A query-based architecture ensures that repeated checks on unchanged code return instantly from cache.

## Self-Hosting & Configuration
- Install via pip or download pre-built binaries for macOS, Linux, and Windows
- Configure via pyproject.toml or a dedicated pyrefly.toml config file
- Set strict or gradual typing modes depending on project maturity
- Exclude directories or suppress specific error codes via configuration
- VS Code extension available for one-click setup

## Key Features
- Written in Rust for speed, checking large codebases in seconds
- Full Language Server Protocol support for rich IDE integration
- Incremental analysis re-checks only affected files on save
- Supports modern Python typing features including ParamSpec and TypeVarTuple
- Developed and battle-tested at Meta's scale

## Comparison with Similar Tools
- **mypy** — the original Python type checker; Pyrefly is significantly faster due to its Rust implementation
- **Pyright** — Microsoft TypeScript-based checker; Pyrefly offers comparable speed with a different type inference approach
- **Ruff** — fast Python linter; Ruff handles style and lint rules while Pyrefly focuses on type correctness
- **Pyre** — Meta's earlier Python type checker; Pyrefly is the next generation with a rewritten core
- **Pylance** — VS Code extension by Microsoft; Pyrefly is open source and editor-agnostic via LSP

## FAQ
**Q: How does Pyrefly relate to Pyre?**
A: Pyrefly is Meta's next-generation type checker, rewritten from scratch in Rust to replace the OCaml-based Pyre with better performance and maintainability.

**Q: Does it support gradual typing?**
A: Yes. You can adopt type annotations incrementally. Pyrefly analyzes annotated code and infers types for unannotated code where possible.

**Q: Which Python versions are supported?**
A: Pyrefly supports Python 3.8 and later, including the latest typing features.

**Q: Can I use it alongside mypy or Pyright?**
A: Yes. Pyrefly can run in parallel with other type checkers, though results may differ on edge cases.

## Sources
- https://github.com/facebook/pyrefly
- https://pyrefly.org

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Source: https://tokrepo.com/en/workflows/pyrefly-fast-python-type-checker-language-server-meta-419e637d
Author: AI Open Source