Alternative type checker support

[abelcheung]

Todo

• Ty (on hold)
  • Too new e.g. Generic protocol support isn't complete yet support generic protocols in typevar solving astral-sh/ty#1714
• Pyrefly (done!)
  • Quirk: version ≤ 0.46.2 ignores typing_extensions, version ≥ 0.47 breaks JSON output

Evaluation of type checker situations

Ty

Very new but fast and promising, though not ready for production use yet. Currently added support but disabled explicitly during runtime tests.

Vendor	Astral
OS	Windows, Macos, Linux
Written in	Rust
Can validate .pyi	Yes

Pyrefly (implementation complete)

Successor of Pyre, fast like all other Rust-based type checkers. Needs a bunch of type-ignore comments, but otherwise almost ready for production use.

Vendor	Meta
OS	Windows, Macos, Linux
Written in	Rust
Can validate .pyi	Yes

Zuban (plan shelved for now)

• Unusable unless using most simplistic installation layout under Windows. Situation is better under Linux and MacOS. Panic: "Did not find a typeshed folder" when installed via uv with UV_LINK_MODE=symlink zubanls/zuban#240
• Doesn't support typing_extensions
• Doesn't support custom type-ignore prefix and solely relies on 'type: ignore', making it only viable as a mypy substitute, and not good for working together with multiple type checkers

Vendor	Jedi author
OS	Windows, Macos, Linux
Written in	Rust
Can validate .pyi	Yes

Pyre-check (target Pyrefly instead)

The support for PEP561 is new, so the current situation is still not fully evaluated. However some problems are immediately notable:

• Don't understand re-exported symbols (need to import directly from the file it is defined)
• Thinks logging.LoggerAdapter is a non-generic class
• Fail to import external cssselect annotation

Vendor	Facebook
OS	MacOS, Linux
PEP561 support	Yes (according to facebook/pyre-check#70 )
Can validate .pyi	Unknown

PyType (plan shelved for now)

Although not officially supported, typeshed has already been successfully using PyType in test suite by importing pytype.io.parse_pyi.

Vendor	Google
OS	MacOS, Linux
Can validate .pyi	Hidden (via API)

PyCharm (plan shelved for now)

Unfortunately the situation is unsatisfactory despite having PEP561 support. There are multiple unfixed issues involving underscored attributes and methods:

• Autocomplete doesn't work for private attributes of base classes
• Autocompletion not works for function_starting with _

Underscored class names like _Element, _ElementTree are building blocks of lxml, which can't be lived without. Annotation of many function arguments become impossible without using them. But the degree of inconvenience introduced by bugs above seriously hinder UX of developers using lxml. It even induced well-intended yet unnecessary PR on types-lxml.

For now, putting to extreme, PyCharm should be avoided like plague until relevant bugs are fixed.

Vendor	JetBrains
OS	Windows, MacOS, Linux
PEP561 support	Yes

[sakosha]

given lxml 6.0 came out, should we reevaluate pycharm?

[abelcheung]

Maybe, but it's almost at the bottom of my todo list unfortunately. Besides, there won't be any major change until lxml 6.0 is the minimum supported version in this repo.