Query param typed as optional enum does not render as dropdown menu in Swagger's doc

[adrienballsonos]

First Check

• I added a very descriptive title here.
• I used the GitHub search to find a similar question 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 enum import Enum
from typing import Union

from fastapi import FastAPI

app = FastAPI()


class Country(str, Enum):
    FIRST_COUNTRY = "firstCountry"
    SECOND_COUNTRY = "secondCountry"


@app.get("/users")
def find_users(country: Union[Country, None] = None):
    # filter users on country
    print(f"Provided country: {country}")

Description

• Open the Swagger page and check the country query param UI
• I expected to have a dropdown menu with two options (firstCountry, secondContry)
• But I got a text field (see first screenshot)
• Same result if I replace Union[Country, None] by Optional[Country]. I also tested with Country | None by switching to python 3.10 but it didn't work either.

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.105.0

Pydantic Version

2.5.2

Python Version

3.9.9

Additional Context

Here is what I have when I remove the optionality of the query param:

This is what I expected to have, without the red * as the param is optional.

[Kludex]

For now, my recommendation is to do the following:

from enum import Enum

from fastapi import FastAPI
from pydantic.json_schema import SkipJsonSchema

app = FastAPI()


class Country(str, Enum):
    FIRST_COUNTRY = 'firstCountry'
    SECOND_COUNTRY = 'secondCountry'


@app.get('/users')
def find_users(country: Country | SkipJsonSchema[None] = None):
    # filter users on country
    print(f'Provided country: {country}')

There are already multiple discussions about this. FastAPI will solve this at some point.

[adrienballsonos]

My bad, I didn't search hard enough for similar discussions. There is already this one for instance, covering this issue: #10316.

Thanks for your help.

[MasterGroosha]

Using FastAPI 0.111, I can just use country: Country = None and it works fine.