[New-bugs-announce] [issue46491] typing: allow Annotated in outermost scope

[Gregory Beauregard]

New submission from Gregory Beauregard <greg at greg.red>:

Currently, `typing.Annotated` (PEP 593) cannot be used at runtime with `typing.Final` and `typing.ClassVar` with `Annotated` on the outside:

```
from typing import Annotated, Final
# TypeError: typing.Final[int] is not valid as type argument
var: Annotated[Final[int], "foo"] = 4
```

The only tenuously related mention of this I can find in a PEP is in PEP 593 (Annotated) which states "The first argument to Annotated must be a valid type".

I believe the runtime behavior should be changed to allow any ordering for `Annotated` with `ClassVar` and `Final`. This was discussed in the typing-sig PEP 655 thread (TypedDict `Required` and `NotRequired`) where the current plan is to allow `Required`/`NotRequired` in any nesting order with `Annotated` while suggesting the `ClassVar`/`Final` ordering restriction be lifted: https://mail.python.org/archives/list/typing-sig@python.org/message/22CJ5TJGIJ563D6ZKB7R3VUZXTZQND5X/

The argument for doing so is on the mailing list: https://mail.python.org/archives/list/typing-sig@python.org/message/MPMOIBX3XFXCD4ZNDC6AV4CLSI5LN544/

To summarize: adopting an overly strict view of what constitutes a valid type for `Annotated` creates difficulties for people who use runtime introspection of `Annotated` annotations by requiring them to parse additional typing or field annotations (https://bugs.python.org/msg411067). This needlessly exacerbates tension between typing and non-typing uses of annotation space. In order to be a good citizen to other annotation users, the `Annotated` runtime ordering restriction should be lifted.

----------
messages: 411404
nosy: GBeauregard
priority: normal
severity: normal
status: open
title: typing: allow Annotated in outermost scope
type: behavior
versions: Python 3.10, Python 3.11, Python 3.9

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<https://bugs.python.org/issue46491>
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