pyrefly

Pyrefly Plugin

Pyrefly is a fast Python type checker and language server (Rust, Meta). ~15x faster than mypy/pyright; default checker for Instagram's 20M-line codebase. Also ships Pydantic/Django/pytest support and IDE features via pyrefly lsp.

Commands

Type Checking

• pyrefly check run — Type check project (--output-format json for pipelines)
• pyrefly self version — Print pyrefly version

Adoption & Migration

• pyrefly config init — Create config, optionally migrate mypy/pyright settings
• pyrefly suppress run — Add suppression comments to silence existing errors
• pyrefly infer run — Insert inferred type annotations into source
• pyrefly config dump — Show merged configuration
• pyrefly _ _ — Passthrough to pyrefly CLI

Usage Examples

• "Type check this Python project"
• "Migrate this repo from mypy to pyrefly"
• "Silence all existing type errors so CI passes"
• "Add inferred type annotations to this file"
• "Give me type errors as JSON"

Installation

pip install pyrefly
# or: uv tool install pyrefly
# or: cargo install pyrefly

Examples

# Check whole project
pyrefly check

# JSON output for tooling
pyrefly check --output-format json

# Baseline existing errors, then fix incrementally
pyrefly init
pyrefly suppress

# Generate annotations from inference
pyrefly infer path/to/file.py

Key Features

• 1.85M+ lines/sec; sub-10ms rechecks in the IDE
• Drop-in migration: pyrefly init reads mypy.ini / pyrightconfig
• pyrefly suppress baselines existing errors on upgrade
• pyrefly infer writes inferred annotations into source
• Built-in Pydantic, Django, and pytest understanding