What is pyrefly?

Pyrefly is a tool from Meta that checks Python code for type errors before the code runs. In Python, variables and functions can optionally carry type annotations that describe what kind of data they hold or return. A type checker reads those annotations and warns you when the code does something that conflicts with them, catching whole categories of bugs without executing a single line. The main selling point is speed. Pyrefly checks over 1.85 million lines of Python code per second, which the README says is 15 times faster than the two most established alternatives, Mypy and Pyright. At Meta, it is the default type checker for Instagram's Python codebase, which runs to about 20 million lines. Large open source projects including PyTorch and JAX have also adopted it. Beyond checking for errors at the command line, Pyrefly works as a language server, which means it can power editor features like autocomplete, jump-to-definition, hover documentation, and code highlighting. Extensions are available for VS Code, Neovim, and Zed. The check that runs after you save a file typically completes in under 10 milliseconds, so the feedback appears almost immediately while you work. For teams migrating from an existing type checker, Pyrefly provides a few helper commands. One initializes a configuration file and sets up a project. Another adds suppression comments to any errors that already exist in the codebase, so you can start enforcing types on new code without having to fix every old error at once. A third command can infer and insert type annotations automatically for code that does not have them yet. The tool also has built-in understanding of popular Python frameworks like Pydantic and Django, including their specific patterns for defining data models and fields.