Can not define `Final` class var in BaseModel

[zen-xu]

Checks

• I added a descriptive title to this issue
• I have searched (google, github) for similar issues and couldn't find anything
• I have read and followed the docs and still think this is a bug

Bug

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

             pydantic version: 1.8.1
            pydantic compiled: False
                 install path: /Users/zen-xu/.pyenv/versions/3.9.1/lib/python3.9/site-packages/pydantic
               python version: 3.9.1 (default, Mar  3 2021, 21:05:29)  [Clang 12.0.0 (clang-1200.0.32.29)]
                     platform: macOS-11.2.3-arm64-arm-64bit
     optional deps. installed: ['email-validator', 'typing-extensions']

from typing import Final, ClassVar
from pydantic import BaseModel

class Template(BaseModel):
    kind: Final[ClassVar[str]] = "xxx"
    content: str

Will raise TypeError: typing.ClassVar[str] is not valid as type argument

[uriyyo]

As PEP 591 say:

Type checkers should infer a final attribute that is initialized in a class body as being a class variable. Variables should not be annotated with both ClassVar and Final.

I believe you should use only Final in this case.

[uriyyo]

@zen-xu As far as currently pydantic does not support Final you can define mixin class with Final field:

from typing import Final
from pydantic import BaseModel


class KindMixin:
    kind: Final[str] = "xxx"


class Template(KindMixin, BaseModel):
    content: str