Can't use frozen native dataclass

[dimaqq]

First Check

• I added a very descriptive title to this issue.
• I used the GitHub search to find a similar issue and didn't find it.
• I searched the FastAPI documentation, with the integrated search.
• I already searched in Google "How to X in FastAPI" and didn't find any information.
• I already read and followed all the tutorial in the docs and didn't find an answer.
• I already checked if it is not related to FastAPI but to Pydantic.
• I already checked if it is not related to FastAPI but to Swagger UI.
• I already checked if it is not related to FastAPI but to ReDoc.

Commit to Help

• I commit to help with one of those options 👆

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.

[jonatasoli]

In this case I think the response_model statement expect a pydantic object and not a dataclass.

[dimaqq]

Note that dataclasses.dataclass can be used if they are not frozen.
My gut tells me that pydantic attempts to convert my dataclass into their kind in order to generate the schema and/or to validators, in which case this is not a FastAPI problem per se.

If this is indeed a limitation, let's note this in FastAPI docs 🙏🏿

[jonatasoli]

Why in your case, use a dataclass instead of a pydantic object?

[dimaqq]

A couple of reasons, though perhaps neither is super important:

1. simplicity, if I can use stdlib dataclass, why not?
2. exporting data in a shape that's defined in a 3p package

[dimaqq]

I wonder if pydantic/pydantic#3469 is related.
Conceivably, generating the OpenAPI schema for the entire API could use a BaseModel somewhere in pydantic 🤔

[dimaqq]

Or maybe this: pydantic/pydantic#2555

[dimaqq]

Or this? pydantic/pydantic#2065

[jonatasoli]

It looks like something would be more related to pydantic than actually fastapi from the data examples.

[dimaqq]

Indeed it is. All I'm proposing is for FastAPI to document this limitation for the time being.

[dimaqq]

OT: looks like my issue comments are breaking GHA:

github.GithubException.RateLimitExceededException: 403 {"message": "API rate limit exceeded for installation ID 3370073.", "documentation_url": "https://docs.github.com/rest/overview/resources-in-the-rest-api#rate-limiting"}
https://github.com/tiangolo/fastapi/runs/6257027940?check_suite_focus=true

[haf]

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

[haf]

pydantic/pydantic#2557