incompatible type error on CoreSchema by mypy

[ytoku]

PR #528 added a workaround for mypy's performance problem.
But the workaround code causes the incompatible-type error when we assign a concrete schema to a CoreSchema variable or argument.

schema: core_schema.CoreSchema = core_schema.str_schema()

This is particularly problematic in defining a custom type.

code

from typing import Any

from pydantic_core import core_schema


class MyValidationType:
    @classmethod
    def __get_pydantic_core_schema__(
        cls,
        **_kwargs: Any,
    ) -> core_schema.AfterValidatorFunctionSchema:
        return core_schema.field_after_validator_function(
            function=lambda _s, _info: MyValidationType(),  # some validation
            schema=core_schema.str_schema(),  # emits incompatible-type error
        )

result

% pipenv install mypy "pydantic>=2.0a1"
% pipenv run mypy test.py
test.py:14: error: Argument "schema" to "field_after_validator_function" has incompatible type "StringSchema"; expected "Dict[str, Any]"  [arg-type]
Found 1 error in 1 file (checked 1 source file)

environment

% pipenv run pip list
Package           Version
----------------- -------
annotated-types   0.4.0
mypy              1.2.0
mypy-extensions   1.0.0
pip               23.0.1
pydantic          2.0a1
pydantic_core     0.23.1
setuptools        67.5.1
tomli             2.0.1
typing_extensions 4.5.0
wheel             0.38.4

% pipenv run python --version
Python 3.10.6

[ytoku]

I suggest to use Mapping[str, Any] instead of Dict[str, Any].

This possibly causes another problem if a user try to modify the schema,
but in most case, modifying a union-typed variable is done after narrowing (or cast).

[samuelcolvin]

if Mapping[str, Any] mollifies mypy, I'm good with it.

@dmontagu @adriangb?

[adriangb]

I'm okay with that.

@ytoku our general stance is going to be that if you want to use pydantic-core you are probably going to need to use pyright as your type checker.

[dmontagu]

Mapping[str, Any] seems like the right thing anyway as we generally treat core schemas as immutable

[samuelcolvin]

@ytoku would you like to create a PR?

[ytoku]

I tried pyright but I found some compatibility problems. It is difficult to migrate to pyright quickly...

I'll create a PR. Thanks.