It is working right?

[gabrielustosa]

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Example Code

class QueryParams(BaseModel):
    test: str

    @field_validator('test')
    def check_test_starts_with(cls, v):
        if not v.startswith("test"):
            raise ValueError('O campo "test" deve começar com "test"')
        return v

@app.get('/test')
def test(params: QueryParams = Depends()):
   ...

Description

I have an endpoint that uses a Pydantic model for query parameters. This model includes validation for one of its fields. However, instead of returning a 422 invalid request error (as it would when using a Pydantic model to validate the request body), it returns a 500 error. Is it supposed to work like this?

Operating System

Windows

Operating System Details

No response

FastAPI Version

0.110.0

Pydantic Version

2.6.4

Python Version

Python 3.11.0rc1

Additional Context

No response

[khaledadrani]

You can work around this by creating a custom exception and adding its custom handler to your fastapi application.
Check this stackoverflow question for more information.

from fastapi import FastAPI, Depends
from pydantic.v1 import BaseModel, validator
from fastapi.requests import Request
from fastapi.responses import JSONResponse


class BadQueryParam(Exception):
    pass


class QueryParams(BaseModel):
    test: str

    @validator('test')
    def check_test_starts_with(cls, v):
        if not v.startswith("test"):
            raise BadQueryParam('O campo "test" deve começar com "test"')
        return v


app = FastAPI()


@app.exception_handler(BadQueryParam)
async def value_error_exception_handler(request: Request, exc: BadQueryParam):
    return JSONResponse(
        status_code=422,
        content={"message": str(exc)},
    )


@app.get('/test')
def test(params: QueryParams = Depends()):
    return {"test": params.test}


if __name__ == "__main__":
    import uvicorn

    uvicorn.run("app:app", host="localhost", port=8000)

[YuriiMotov]

I believe it shouldn't return 500 error, but now it really does.

But if you use Field's pattern parameter instead of field_validator(which is IMO better in this particular case), it works fine:

from fastapi import Depends, FastAPI
from pydantic import BaseModel, Field

app = FastAPI()

class QueryParams(BaseModel):
    test: str = Field(pattern=r'^test')

@app.get('/test')
def test(params: QueryParams = Depends()):
   ...

[YuriiMotov]

Reviewed this one more time.

Since v0.115.0 FastAPI officially supports Pydantic models in Query parameters (and in Header and Cookie as well).

So, after updating the code according to new docs, it works well:

from typing import Annotated
from fastapi import FastAPI, Query
from pydantic import BaseModel, field_validator


class QueryParams(BaseModel):
    test: str

    @field_validator("test")
    def check_test_starts_with(cls, v):
        if not v.startswith("test"):
            raise ValueError('O campo "test" deve começar com "test"')
        return v


app = FastAPI()


@app.get("/test")
def test(params: Annotated[QueryParams, Query()]):
    pass