A veritable garden of linters and type checkers has blossomed in the Python ecosystem. Pyright and Mypy are two of the most popular, while Astral’s ty and Meta’s Pyrefly are two promising Rust-based newcomers. We compared early-stage ty and Pyrefly last June. One year later, Meta has released Pyrefly 1.0.
Pyrefly is intended to stand out from the pack by way of a few key features. It’s written in Rust for performance and memory efficiency, and it has a number of intriguing forward-looking features, some still experimental.
Setting up Pyrefly
Pyrefly installs into a Python environment like any other Python type checker (pip install pyrefly) and brings with it no additional dependencies. Visual Studio Code users can work with Pyrefly via an extension, but the extension doesn’t give you access to the command-line tools offered by Pyrefly when it’s installed in a project venv.
You can start using Pyrefly right away, with no actual setup. If Pyrefly detects no configuration for a given project, it defaults to flagging only the most high-profile errors: syntax issues, unrecognized names, and so on. To start using Pyrefly’s more advanced features, you can add a [tool.pyrefly] section in pyproject.toml, or create a pyrefly.toml file, and populate those with your desired settings.
You can also run pyrefly init on your codebase and set things up automatically, including migrating existing settings from previous Pyrefly installs or from Mypy or Pyright. By default Pyrefly will use **/*.py* as the project’s includes directory, and will automatically ignore site package paths from your configured environment, thus sparing you potentially thousands of spurious errors from your venv’s installed packages.
Using Pyrefly
Once you have Pyrefly set up on a project, you’ll likely see an immense number of new errors. To silence all of them at once via # pyrefly: ignore comment directives, run pyrefly suppress. This will not only reduce the amount of error noise, but allow you to see what each error actually is from its suppression comments.
Suppression comments have another nice readability feature: they can be placed on the line before the offending error, rather than at the end of it. You can also pre-emptively suppress errors for an entire module with one directive at the top of the file, but having each error suppressed individually makes more sense. And while there’s a pyrefly suppress --remove-unused command to remove suppressions that have no actual error, there’s no command for stripping a codebase of all suppressions. However, because suppressions are consistently formatted, you could remove those suppressions with a simple regex-powered search-and-replace.
A currently experimental feature called baseline files borrows an idea from the basedpyright tool. You generate a project-wide .json file with pyrefly check --baseline that tracks all existing errors in the project, then only new errors apart from those will be reported. It’s a handy way to refactor a codebase when new work is being done on it, so that existing problems can be handled separately.
If you have the Pyrefly extension installed in VS Code, objects without existing type hints will have suggested type hints displayed as inlays if the types can be inferred from the code. These can be added with a right click, or inserted automatically across the codebase with the command pyrefly infer.
Advanced Pyrefly features
Two major third-party Python packages have direct type annotation support in Pyrefly. One is Django and its ORM, via the django-stubs package. Models, fields, relationships, class-based generic views, and many other features have typing support. The other package, Pydantic, gets support from Pyrefly for features like fields, dataclasses, and Pydantic’s own runtime validation logic. Pydantic validation can be “lax” (automatic coercion of types via unions) or “strict” (exact types required).
Many advanced Pyrefly features are still considered experimental, but have a lot of promise. With type coverage, for instance, you can use pyrefly report to generate a JSON report of how thoroughly types are used in the codebase. (This is not to be confused with the coverage package in Python, which describes test coverage for a codebase.)
Another experimental feature lets you generate stub files for a codebase by using static code analysis (not runtime analysis). PyTorch users also benefit from a form of experimental Pyrefly support. Tensor shapes can be described with types and type-checked across transformations. Note that the typing used for this is exclusive to Pyrefly, as there is no agreed-on standard for how to do this.
Pyrefly vs. Pyright and Mypy
Pyright and Mypy have been the default choices for linting and type checking in the Python world for some time now. Pyright is an easy default for Visual Studio Code users, as it’s part of Microsoft’s Python extensions for that editor, and Mypy offers extensibility and broader compatibility with older codebases. Pyrefly, on the other hand, is forward-looking, with fewer legacy concerns. It also has a higher score for test conformance than Mypy, though not a higher score than Pyright.
Other key differences include the type checking philosophy used by each, and their licensing. Pyrefly and Pyright infer as aggressively as possible, while Mypy’s default is to skip anything unannotated. And while Pyrefly and Mypy are MIT-licensed, only Pyright’s core functionality is MIT-licensed. Pylance, the language server extension for VS Code that is powered by Pyright, is proprietary.
Finally, Pyrefly’s Rust codebase gives it a default performance edge over both Mypy and Pyright, which are written in Python and TypeScript, respectively. Astral’s ty, also written in Rust, is still more prototype than mature project. Pyrefly has enough of a feature set right now that it’s already worth experimenting with—although that’s best done on a branch of your code where adding type annotations, suppression comments, and init files won’t pose a problem.
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