NonGNU ELPA - flymake-pyrefly

Pyrefly is a fast Python type checker written in Rust. flymake-pyrefly is a Pyrefly backend for Flymake that helps you to see typing errors in your Python files.

flymake-pyrefly is available on nonGNU Emacs Lisp Package Archive, so you can install it by M-x package-install RET flymake-pyrefly RET.

To add flymake-pyrefly to the list of Flymake backends when editing Python files (notice the difference if you use eglot):

(add-hook 'python-base-mode-hook #'pyrefly-setup-flymake-backend)

;; alternatively, with use-package
(use-package flymake-pyrefly
  :hook (python-base-mode . pyrefly-setup-flymake-backend))

;; or if you use eglot
(add-hook 'eglot-managed-mode-hook #'pyrefly-setup-flymake-backend)

;; if you use eglot and use-package
(use-package flymake-pyrefly
  :hook (eglot-managed-mode . pyrefly-setup-flymake-backend))

If Pyrefly binary is not visible from current Python virtual environment, you can set it through the interface M-x customize-variable RET flymake-pyrefly-binary-file RET.