Pyrefly

0.60.2 · active · verified Fri Apr 10

Pyrefly is a fast type checker and language server for Python, developed by Meta and written in Rust. It provides lightning-fast static type checking along with powerful IDE features such as code navigation, semantic highlighting, and code completion. Pyrefly aims to catch type-related errors before runtime, improve code quality, and enhance the developer experience. It is available as a command-line tool and an IDE extension, with new releases typically every Monday and more frequent updates for new features and bug fixes.

Warnings

breaking The `pyrefly-pre-commit` hook underwent significant changes from version 0.0.1 to 0.42.0. Users migrating from older versions need to update their pre-commit configuration.
Fix: Migrate your `.pre-commit-config.yaml` to use the consolidated `pyrefly-check` hook. Refer to the `pyrefly-pre-commit` GitHub repository for specific migration steps.
gotcha When upgrading Pyrefly (or its dependencies), new type errors might appear due to improved analysis or stricter checks. Addressing all at once can be impractical.
Fix: Use `pyrefly check --suppress-errors` to temporarily silence existing errors, followed by `pyrefly check --remove-unused-ignores` after running your code formatter. Repeat until a clean state is achieved.
gotcha The `pyrefly init` command may report a 'No Python files matched patterns' error if run in an empty directory. This is expected behavior and will resolve once Python files are added to the project.
Fix: This is not an actual error, but an informational message. Proceed to add your Python or Jupyter Notebook files to the project.
gotcha Support for integrating with specific build systems (e.g., Buck2) is currently unstable and may break without prior notice. These issues are generally lower priority than core type-checking functionality.
Fix: Be aware of potential instabilities if relying on Pyrefly's build system integration. Monitor official documentation and GitHub issues for updates and workarounds.

Install

pip install pyrefly Pip
conda install -c conda-forge pyrefly Conda

Quickstart

This quickstart demonstrates how to install Pyrefly, initialize a project, and run a basic type check on a Python file containing a simple type error. Pyrefly operates as a command-line tool, not typically imported within Python code.

# 1. Create a new project directory and navigate into it
# mkdir my_project && cd my_project

# 2. Install Pyrefly (assuming pip is installed and virtual environment is active)
# pip install pyrefly

# 3. Initialize Pyrefly configuration (creates pyproject.toml or pyrefly.toml)
# pyrefly init

# 4. Create a sample Python file with a type error (e.g., 'main.py')
# echo 'def greet(name: str) -> str:\n    return f"Hello, {name}"\n\nprint(greet(42))' > main.py

# 5. Run Pyrefly to check for type errors
# pyrefly check

# Expected output for 'print(greet(42))' should show a type error, e.g.:
# main.py:3:11: Type mismatch: Expected `str` but got `int`

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