Returning `dataclass` with pydantic-compatible fields (e.g. datetime) fails for FastAPI >= 0.67

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Example & Description

I have encountered an issue when returning dataclass responses with some pydantic-compatible (but incompatible with json encoder of Python standard library) fields, e.g. datetime objects or pathlib file paths.

Here’s a self-contained, minimal, reproducible, example where I have added a datetime field to the Item class in tests/test_serialize_response_dataclass.py:

from datetime import datetime
from fastapi import FastAPI
from fastapi.testclient import TestClient
from dataclasses import dataclass

app = FastAPI()


@dataclass
class Item:
    name: str
    date: datetime


@app.get("/items/no-response-model/object")
def get_no_response_model_object():
    return Item(name="object", date=datetime(2021, 7, 26))


client = TestClient(app)


def test_no_response_model_object():
    response = client.get("/items/no-response-model/object")
    response.raise_for_status()
    assert response.json() == {
        "name": "object",
        "date": datetime(2021, 7, 26).isoformat(),
    }

I would

• Run the test code snippet with Pydantic 0.67
• I would expect it pass because datetime objects are compatible with Pydantic models
• It fails with the following stack trace:

Fail: TypeError: Object of type datetime is not JSON serializablepytest(./tests/test_serialize_response_dataclass_minimal_failing.py::test_no_response_model_object)
test_serialize_response_dataclass_minimal_failing.py(23, 5): test_no_response_model_object
test_serialize_response_dataclass_minimal_failing.py(24, 5): response = client.get("/items/no-response-model/object")
sessions.py(555, 9): return self.request('GET', url, **kwargs)
testclient.py(415, 9): return super().request(
sessions.py(542, 9): resp = self.send(prep, **send_kwargs)
sessions.py(655, 9): r = adapter.send(request, **kwargs)
testclient.py(243, 17): raise exc from None
testclient.py(240, 13): loop.run_until_complete(self.app(scope, receive, send))
base_events.py(642, 9): return future.result()
applications.py(199, 17): await super().__call__(scope, receive, send)
applications.py(112, 9): await self.middleware_stack(scope, receive, send)
errors.py(181, 13): raise exc from None
errors.py(159, 13): await self.app(scope, receive, _send)
exceptions.py(82, 17): raise exc from None
exceptions.py(71, 13): await self.app(scope, receive, sender)
routing.py(580, 17): await route.handle(scope, receive, send)
routing.py(241, 13): await self.app(scope, receive, send)
routing.py(52, 13): response = await func(request)
routing.py(243, 13): response = actual_response_class(response_data, **response_args)
responses.py(53, 9): self.body = self.render(content)
responses.py(161, 9): return json.dumps(
__init__.py(234, 5): return cls(
encoder.py(199, 9): chunks = self.iterencode(o, _one_shot=True)
encoder.py(257, 9): return _iterencode(o, 0)
encoder.py(179, 9): raise TypeError(f'Object of type {o.__class__.__name__} '

I have created a PR #3607 with an attempt to fix the issue and extend the existing tests in tests/test_serialize_response_dataclass.py. Without this PR, all tests in this updated file fail.

Note that the example above and the extended tests also pass with a pre 0.67 version of FastApi (before official dataclass response support).

Environment

• OS: [e.g. Linux / Windows / macOS]: macOS
• FastAPI Version [e.g. 0.3.0]: 0.67.0
• Python version: 3.9.6

[tiangolo]

Thanks for the complete report! And thanks for the fix @himbeles in https://github.com/tiangolo/fastapi/pull/3607

This will be available in the next release, FastAPI 0.81.0 🚀 🎉

[dragoon]

This is also important for me, and the PR already exists, let me know if you need any help on this.