Query parameter type is not instantiated using __new__

[chbndrhnns]

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Example Code

from datetime import datetime

import fastapi
import httpx
import pytest
from fastapi import FastAPI

app = FastAPI()


class CustomDate(datetime):
    def __new__(cls, *args, **kwargs):
        try:
            parsed = datetime.strptime(args[0], "%Y-%m-%dT%H:%M:%S.%f%z")
            return super().__new__(cls, parsed.year + 1, month=parsed.month,
                                   day=parsed.day)
        except TypeError:
            return super().__new__(cls, *args, **kwargs)


@app.get("/")
async def get(begin: CustomDate = fastapi.Query()):
    return {"data": begin}


@pytest.fixture
def anyio_backend():
    return "asyncio"




def test_can_create_CustomDate():
    timestamp = "2022-10-20T18:25:43.511Z"
    assert str(CustomDate(timestamp)).startswith("2023")


@pytest.mark.anyio
async def test_():
    timestamp = "2022-10-20T18:25:43.511Z"

    async with httpx.AsyncClient(app=app, base_url="http://127.0.0.1") as client:
        res = await client.get("/", params={"begin": timestamp})
        assert res.status_code == 200
        assert res.json()["data"].startswith("2023")

Description

I have a simple subclass of datetime.datetime with customized parsing. (Please ignore the specifics, I just wanted to highlight the problem). The __new__ method of my subclass is not being called which means the customized parsing is not happening when using the type as an annotation on a query parameter.

The reason why I am doing it is to make use of the FastAPI validations and not needing to validate the value further down in my code. I can re-create the instance manually to trigger the __new__ method and then fail manually but this feels strange.

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.89.1

Python Version

3.9.6

Additional Context

No response

[yinziyan1206]

It's better to use DI to create specific type

[chbndrhnns]

It's better to use DI to create specific type

Better how? At least the current behaviour is unexpected to me.