Can I use experimental types from typing_extensions?

Question

To be more specific:

To solve questions like How do I type hint a method with the type of the enclosing class?
PEP 673 introduces typing.Self. The PEP is a Draft, but it currently an experimental type in typing_extensions 4.0.0

I tried using this in python 3.8

@dataclasses.dataclass
class MenuItem:
    url: str
    title: str
    description: str = ""
    items: typing.List[typing_extensions.Self] = dataclasses.field(default_factory=list)

But it raises

TypeError: Plain typing_extensions.Self is not valid as type argument

I could just use the literal string "MenuItem" instead. But I was wondering why this doesn't work.

This looks like a bug in `typing_extensions`, TBH. The metatype of ``Self`` is usually used for "non types" such as ``Union``, ``Literal`` or ``NoReturn``; its only actual type ``Any`` is special cased by ``typing``. — MisterMiyagi, Nov 23 '21 at 15:54
"It will be in Python 3.11" — not necessarily! The PEP hasn't even been formally submitted to the Steering Council for consideration yet, let alone approved! You can see its status is still given as "draft": https://www.python.org/dev/peps/pep-0673/. (To be clear, I support the PEP, and hope it is accepted.) — Alex Waygood, Nov 27 '21 at 23:17
The PEP is now accepted, but I'm still getting the same error. `typing-extensions==4.3.0`, `mypy==0.971`, `python3.10.5`. — Maxxik CZ, Aug 06 '22 at 18:51
`pyright` correctly recognizes `Self` as a type, so the issue is probably in mypy. — Maxxik CZ, Aug 07 '22 at 08:59
As of today, `Self` is not implemented in mypy yet. Currently it's being worked on in this [PR](https://github.com/python/mypy/pull/13133). — Maxxik CZ, Aug 07 '22 at 09:28
This commit was merged on 2022-11-15: [`77dd4b4d`](https://github.com/python/mypy/commit/77dd4b4df5b8bcd716352144feeb78862427f4dd) but as of today, is not part of any tagged release of mypy. — Jonathon Reinhart, Jan 18 '23 at 17:25

Chris Wesseling · Accepted Answer · 2022-09-22T14:22:38.200

2

Yes you can, but be aware of the uses of the package:

The typing_extensions module serves two related purposes:

Enable use of new type system features on older Python versions. For example, typing.TypeGuard is new in Python 3.10, but typing_extensions allows users on previous Python versions to use it too.

Enable experimentation with new type system PEPs before they are accepted and added to the typing module.

This specific case was a bug in typing_extensions. It's being planned to get fixed in 4.0.1.

edited Sep 22 '22 at 14:22

answered Nov 29 '21 at 09:16

Chris Wesseling

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I have version 4.3.0 and am still getting the error. — edit: my error message is actually `Variable "typing_extensions.Self" is not valid as a type` which points to it being an issue in mypy, not (anymore) in typing_extensions. The pull request to fix this is still open as of the time of writing: https://github.com/python/mypy/pull/11666 – theberzi Aug 14 '22 at 12:23
@theberzi Yeah, that was also [mentioned in the comments on the question](https://stackoverflow.com/questions/70083371/can-i-use-experimental-types-from-typing-extensions/70152211?noredirect=1#comment129392100_70083371) – Chris Wesseling Aug 20 '22 at 09:49

Can I use experimental types from typing_extensions?

1 Answers1