GenericModel[Any] raises ValidationError for GenericModel[SomeModel]

[gsakkis]

Initial Checks

• I confirm that I'm using Pydantic V2

Description

Annotating a field as ResponseModel[Any] (where ResponseModel is a generic BaseModel) fails to validate values of type ResponseModel and ResponseModel[Product]; it only validates values of ResponseModel[Any].

On the other hand annotating the field as ResponseModel validates all three cases, however mypy gives Missing type parameters for generic type "ResponseModel".

Questions:

• Is the ResponseModel[Any] validation working as expected or it's a bug?
• Is it safe to # type: ignore the second version that seems to be working?

Example Code

from typing import Any, Generic, TypeVar

from pydantic import BaseModel, ValidationError

T = TypeVar("T")


class ResponseModel(BaseModel, Generic[T]):
    content: T


class Product(BaseModel):
    name: str
    price: float


class Order(BaseModel):
    id: int
    product: ResponseModel[Any]
    # replacing the previous annotation with the following succeeds at runtime but fails type checking
    # product: ResponseModel


product = Product(name="Apple", price=0.5)
response1: ResponseModel[Any] = ResponseModel[Any](content=product)
response2: ResponseModel[Any] = ResponseModel(content=product)
response3: ResponseModel[Any] = ResponseModel[Product](content=product)

for response in response1, response2, response3:
    try:
        order = Order(id=1, product=response)
        print(f"{response!r} succeeded")
    except ValidationError:
        print(f"{response!r} failed")

Output:

ResponseModel[Any](content=Product(name='Apple', price=0.5)) succeeded
ResponseModel(content=Product(name='Apple', price=0.5)) failed
ResponseModel[Product](content=Product(name='Apple', price=0.5)) failed

Python, Pydantic & OS Version

pydantic version: 2.7.1
        pydantic-core version: 2.18.2
          pydantic-core build: profile=release pgo=true
               python version: 3.11.8 | packaged by conda-forge | (main, Feb 16 2024, 20:49:36) [Clang 16.0.6 ]
                     platform: macOS-14.1-arm64-arm-64bit
             related packages: mypy-1.9.0 typing_extensions-4.10.0 pydantic-settings-2.2.1
                       commit: unknown

[sydney-runkle]

Hi @gsakkis,

Thanks for your questions. This comment might help to add some context regarding some of the nuances with validation and parametrized generics: #8884 (comment).

The case with Any is interesting. This does seem a bit buggy to me. Specifically, this works:

from typing import Any, Generic, TypeVar

from pydantic import BaseModel, ValidationError

T = TypeVar("T")


class ResponseModel(BaseModel, Generic[T]):
    content: T


class Product(BaseModel):
    name: str
    price: float


class Order(BaseModel):
    id: int
    product: ResponseModel[Any]
    # replacing the previous annotation with the following succeeds at runtime but fails type checking
    # product: ResponseModel


product = Product(name="Apple", price=0.5)
response1: ResponseModel[Any] = ResponseModel[Any](content=product)
response2: ResponseModel[Any] = ResponseModel(content=product)
response3: ResponseModel[Any] = ResponseModel[Product](content=product)

for response in response1, response2, response3:
    try:
        order = Order(id=1, product=response.model_dump())
        print(f"{response!r} succeeded")
    except ValidationError:
        print(f"{response!r} failed")

I'm going to chat with @dmontagu, our resident generics expert, about this and will get back to you!