[BUG] Generic pydantic model causes inconsistent OpenAPI spec generation

Describe the bug I am wrapping my responses in a generic model like below (full code in toggle section).

class Response(GenericModel, Generic[DataT]):
    """Wrapper for responses"""
    data: Optional[DataT]

I see two issues:

• The /docs endpoint renders things differently for a contained list or a contained single element.
• PyCharm warns on type mismatch according to the annotations

I am wondering if I am using the GenericModel in a wrong way or if there is a bug?

Click to toggle

from typing import TypeVar, Generic, Optional, List

import uvicorn
from fastapi import FastAPI
from pydantic import BaseModel
from pydantic.generics import GenericModel

DataT = TypeVar('DataT')


class Response(GenericModel, Generic[DataT]):
    """Wrapper for responses"""
    data: Optional[DataT]


class ProjectOut(BaseModel):
    """Project model (out)"""
    id: int


app = FastAPI()

out = ProjectOut(id=1)


@app.get(
    '/projects/{item_id}',
    response_model=Response[ProjectOut]
)
def get_single() -> Response[ProjectOut]:
    return Response[ProjectOut](data=out)


@app.get(
    '/projects',
    response_model=Response[List[ProjectOut]])
def get_projects() -> Response[List[ProjectOut]]:
    return Response[List[ProjectOut]](data=[out])


if __name__ == '__main__':
    uvicorn.run(
        f"{__name__}:app",
        host='0.0.0.0',
        port=8888,
        reload=True,
        debug=True,
        log_level='info')

To Reproduce

Expected behavior

• The list response is shown as Response[List[ProjectOut]]
• I see no warnings in PyCharm when I am using annotations correctly.

Screenshots

Environment:

• OS: macOS
• FastAPI Version: 0.42
• Python version: 3.7.4

Additional context Add any other context about the problem here.

[fhessel]

@KiraPC – I just stumbled upon the same issue as you and Google led me here, so I’ll write down the solution to that:

Instead of deriving your PaginationModel from pydantic.BaseModel you have to use pydantic.generics.GenericModel, so the following gives you the expected result:

from pydantic import BaseModel
from pydantic.generics import GenericModel

Model = TypeVar("Model", bound=BaseModel)

class PaginationModel(GenericModel, Generic[Model]):
    index: int
    size: int
    total: int
    items: List[Model]

[KiraPC]

Hello everyone.

I have a similar issue to the one presented here.

Basically I’m using a pydantic BaseModel to represent the Pagination response. This class extends generic with a TypeVar.

T = TypeVar("T")


class PaginationModel(BaseModel, Generic[T]):
    total: int = Field(description="Total number of elements on DB")
    index: int = Field(description="The current page index")
    size: int = Field(description="The size of the page")
    results: List[T]

Then I use as Generic input the object that should be in the results list

class MyMockModel(BaseModel):
    foo: int
    bar: int

Then use it as route response model

def get_foo_bar(response_model=PaginationModel[MyMockModel]):
     ...

When I print the openapi spec, I expect to have something like

{
    "total": 0,
    "index": 0,
    "size": 20,
    "result": [
        {
            "foo": 0,
            "bar": 0
        }
    ]
}

but what actually I get is

{
   "foo": 0,
   "bar": 0
}

I’m doing somethings wrong, or it is a bug?