No key validation for typed __pydantic_extra__

[RLKRo]

Initial Checks

• I confirm that I'm using Pydantic V2

Description

Adding type annotation for __pydantic_extra__ only validates values of the extra fields.
I expected that annotating __pydantic_extra__ with something like Dict[constr(pattern="..."), str] would lead to validation of extra field names.
In the example below __pydantic_extra__ is annotated as Dict[constr(pattern="extra"), str] which I expected to lead to validation errors for any extra field names that do not contain "extra".

If this behavior is intended, documentation

By default, no validation will be applied to these extra items, but you can set a type for the values by overriding the type annotation for __pydantic_extra__:

should mention that field names are not validated.

Example Code

from typing import Dict
from typing_extensions import Annotated

from pydantic import BaseModel, ValidationError, constr


class ExtraWithIncorrectValue(BaseModel, extra="allow"):
    __pydantic_extra__: Dict[str, int]


# values are validated for extra fields
try:
    ExtraWithIncorrectValue(a="a")
except ValidationError:
    print("ExtraWithIncorrectValue raised an error")


class DictWithIncorrectConstrKey(BaseModel, extra="allow"):
    my_dict: Dict[constr(pattern="extra"), str]


# dict keys are validated
try:
    DictWithIncorrectConstrKey(my_dict={"a": "a"})
except ValidationError:
    print("DictWithIncorrectConstrKey raised an error")


class ExtraWithIncorrectConstrKey(BaseModel, extra="allow"):
    __pydantic_extra__: Dict[constr(pattern="extra"), str]


# extra field names are not validated
ExtraWithIncorrectConstrKey(a="a")
print("ExtraWithIncorrectConstrKey did not raise an error")

Python, Pydantic & OS Version

pydantic version: 2.7.1
        pydantic-core version: 2.18.2
          pydantic-core build: profile=release pgo=true
                 install path: /home/[redacted]/.cache/pypoetry/virtualenvs/[redacted]/lib/python3.12/site-packages/pydantic
               python version: 3.12.3 (main, Apr 23 2024, 09:16:07) [GCC 13.2.1 20240417]
                     platform: Linux-6.8.9-zen1-1-zen-x86_64-with-glibc2.39
             related packages: typing_extensions-4.11.0
                       commit: unknown

[sydney-runkle]

Marking this as a feature request, because I believe we could add key validation support. I do believe that the current docs make it clear that only validation is applied to the values with the current design.

Thanks for the FR. PRs welcome!

[RLKRo]

Would also be great to be able to annotate __pydantic_extra__ as a whole:

    __pydantic_extra__: Annotated[dict[str, Any], MyValidator]

instead of calling MyValidator inside model_validator