Optional query params are broken in swagger when pydantic2 is installed

[zoola969]

First Check

• I added a very descriptive title here.
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• 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.

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Example Code

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
def root(
    q: int | None = Query(None),
):
    return q

Description

Open /docs in browser. I expect optional query parameter with integer type, but there is just common text optional query param
pydantic v2

pydantic v1

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.105.0

Pydantic Version

2.5.3

Python Version

3.11.7

Additional Context

No response

[JavierSanchezCastro]

Use Annotated like

from fastapi import FastAPI, Query
from typing import Annotated

app = FastAPI()


@app.get("/")
def root(q: Annotated[int, Query()] = None):
    return q

[zoola969]

It didn't help me to understand how to correctly define optional query param.
I look at openapi.json data and see the difference in how the schema is generated after switching to pydantic v2
p1:
"parameters":[{"required":false,"schema":{"type":"integer","title":"Q"},"name":"q","in":"query"}]
p2:
"parameters":[{"name":"q","in":"query","required":false,"schema":{"anyOf":[{"type":"integer"},{"type":"null"}],"title":"Q"}}]

So, there is definitely a bug in fastapi or pydantic

[JavierSanchezCastro]

#10781 (reply in thread)

The problem with p1 lies in its allowance of a null value, which is not reflected in the openapi.json. P1 represents the oldest openapi generated schema, and Swagger is capable of displaying this information because there is only one type for the query parameter in the generated schema, even though it permits a null value.

However, with the corrected openapi schema as p2, there are now two types for the same parameter. I suppose Swagger doesn't know how to display that information, but you can see that in /redoc that query parameter would be an integer or null.

That's what I mean by it depends on how those pages decide to render the schema

[zoola969]

Thanks, I got it. And it helps me to find several issues about it and one hack solution.

Using SkipJsonSchema removes anyOf and null from openapi.json and swagger shows parameter correctly

q: Annotated[int | SkipJsonSchema[None], Query()] = None

"parameters":[{"name":"q","in":"query","required":false,"schema":{"type":"integer","title":"Q"}}]

#9709 (reply in thread)
pydantic/pydantic#6653 (comment)

It seems like swagger doesn't support anyOf for parameters

[zoola969]

btw will be nice to see this solution or problem description somewhere in docs, this is a stopper of using p2 for me

[leonidktoto]

@tiangolo q: Annotated[int | SkipJsonSchema[None], Query()] = None I started studying fastapi, I think this solution is quite working. what do you say? Is there another solution to this problem?

[sabonerune]

@tiangolo
By the way, are there any plans to fix this problem in FastAPI?

https://json-schema.org/understanding-json-schema/reference/null

It's important to remember that in JSON, null isn't equivalent to something being absent.

I don't think FastAPI(or maybe OpenAPI) has a way to accept null in queries.