Can't use frozen native dataclass

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Example Code

from dataclasses import dataclass
from fastapi import FastAPI

app = FastAPI()

@dataclass(frozen=True)
class Mod:
    foo: str


@app.get("/", response_model=Mod)
async def get_mod():
    return {"foo": "bar"}

Description

• access http://localhost:8000/openapi.json
• see the crash

ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "/home/dima/.cache/pypoetry/virtualenvs/mre-frozen-dataclass-3kNYbFrL-py3.11/lib/python3.8/site-packages/uvicorn/protocols/http/h11_impl.py", line 366, in run_asgi
    result = await app(self.scope, self.receive, self.send)
  File "/home/dima/.cache/pypoetry/virtualenvs/mre-frozen-dataclass-3kNYbFrL-py3.11/lib/python3.8/site-packages/uvicorn/middleware/proxy_headers.py", line 75, in __call__
    return await self.app(scope, receive, send)
  File "/home/dima/.cache/pypoetry/virtualenvs/mre-frozen-dataclass-3kNYbFrL-py3.11/lib/python3.8/site-packages/fastapi/applications.py", line 261, in __call__
    await super().__call__(scope, receive, send)
  File "/home/dima/.cache/pypoetry/virtualenvs/mre-frozen-dataclass-3kNYbFrL-py3.11/lib/python3.8/site-packages/starlette/applications.py", line 112, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/home/dima/.cache/pypoetry/virtualenvs/mre-frozen-dataclass-3kNYbFrL-py3.11/lib/python3.8/site-packages/starlette/middleware/errors.py", line 181, in __call__
    raise exc
  File "/home/dima/.cache/pypoetry/virtualenvs/mre-frozen-dataclass-3kNYbFrL-py3.11/lib/python3.8/site-packages/starlette/middleware/errors.py", line 159, in __call__
    await self.app(scope, receive, _send)
  File "/home/dima/.cache/pypoetry/virtualenvs/mre-frozen-dataclass-3kNYbFrL-py3.11/lib/python3.8/site-packages/starlette/exceptions.py", line 82, in __call__
    raise exc
  File "/home/dima/.cache/pypoetry/virtualenvs/mre-frozen-dataclass-3kNYbFrL-py3.11/lib/python3.8/site-packages/starlette/exceptions.py", line 71, in __call__
    await self.app(scope, receive, sender)
  File "/home/dima/.cache/pypoetry/virtualenvs/mre-frozen-dataclass-3kNYbFrL-py3.11/lib/python3.8/site-packages/fastapi/middleware/asyncexitstack.py", line 21, in __call__
    raise e
  File "/home/dima/.cache/pypoetry/virtualenvs/mre-frozen-dataclass-3kNYbFrL-py3.11/lib/python3.8/site-packages/fastapi/middleware/asyncexitstack.py", line 18, in __call__
    await self.app(scope, receive, send)
  File "/home/dima/.cache/pypoetry/virtualenvs/mre-frozen-dataclass-3kNYbFrL-py3.11/lib/python3.8/site-packages/starlette/routing.py", line 656, in __call__
    await route.handle(scope, receive, send)
  File "/home/dima/.cache/pypoetry/virtualenvs/mre-frozen-dataclass-3kNYbFrL-py3.11/lib/python3.8/site-packages/starlette/routing.py", line 259, in handle
    await self.app(scope, receive, send)
  File "/home/dima/.cache/pypoetry/virtualenvs/mre-frozen-dataclass-3kNYbFrL-py3.11/lib/python3.8/site-packages/starlette/routing.py", line 61, in app
    response = await func(request)
  File "/home/dima/.cache/pypoetry/virtualenvs/mre-frozen-dataclass-3kNYbFrL-py3.11/lib/python3.8/site-packages/fastapi/applications.py", line 216, in openapi
    return JSONResponse(self.openapi())
  File "/home/dima/.cache/pypoetry/virtualenvs/mre-frozen-dataclass-3kNYbFrL-py3.11/lib/python3.8/site-packages/fastapi/applications.py", line 191, in openapi
    self.openapi_schema = get_openapi(
  File "/home/dima/.cache/pypoetry/virtualenvs/mre-frozen-dataclass-3kNYbFrL-py3.11/lib/python3.8/site-packages/fastapi/openapi/utils.py", line 412, in get_openapi
    flat_models = get_flat_models_from_routes(routes)
  File "/home/dima/.cache/pypoetry/virtualenvs/mre-frozen-dataclass-3kNYbFrL-py3.11/lib/python3.8/site-packages/fastapi/openapi/utils.py", line 377, in get_flat_models_from_routes
    flat_models = callback_flat_models | get_flat_models_from_fields(
  File "pydantic/schema.py", line 448, in pydantic.schema.get_flat_models_from_fields
  File "pydantic/schema.py", line 421, in pydantic.schema.get_flat_models_from_field
  File "pydantic/dataclasses.py", line 255, in pydantic.dataclasses.dataclass
  File "pydantic/dataclasses.py", line 250, in pydantic.dataclasses.dataclass.wrap
  File "pydantic/dataclasses.py", line 159, in pydantic.dataclasses._process_class
  File "/usr/lib/python3.8/dataclasses.py", line 1019, in dataclass
    return wrap(cls)
  File "/usr/lib/python3.8/dataclasses.py", line 1011, in wrap
    return _process_class(cls, init, repr, eq, order, unsafe_hash, frozen)
  File "/usr/lib/python3.8/dataclasses.py", line 891, in _process_class
    raise TypeError('cannot inherit non-frozen dataclass from a '
TypeError: cannot inherit non-frozen dataclass from a frozen one

Operating System

Linux

Operating System Details

Linux

FastAPI Version

0.75.1

Python Version

3.10, 3.11

Additional Context

pydantic 1.9.0

Workarounds:

1. remove frozen=True, or
2. from pyndatic.dataclasses impotr dataclass – the pydantic version works.

It could be a bug in pydantic for all I know, or even a limitation, in which case fastapi docs need to be updated.

[haf]

https://github.com/samuelcolvin/pydantic/pull/2557

[haf]

It seems to be specific to something fastapi might do when it converts normal dataclasses to pydantic dataclasses?