FastAPI - Swagger UI docs not rendering list array as query parameter

Question

According to the docs it is possible to pass a list of values https://fastapi.tiangolo.com/tutorial/query-params-str-validations/#query-parameter-list-multiple-values .

I have tried on my end and the endpoint http://localhost:8000/items/?q=foo&q=bar correctly process the values as a list:

{"q":["foo","bar"]}

But the docs do not, and interpret it as a string and do not render multiple options:

Here is my main.py:

from db import engine,get_db
from models import Example,ExampleModel,Base,Color,Item
from fastapi import FastAPI,Depends,Query,Path
from sqlalchemy.ext.asyncio import AsyncSession
from sqlalchemy import select
from contextlib import asynccontextmanager
from typing import Annotated,Union,List
from fastapi.responses import HTMLResponse


@asynccontextmanager
async def lifespan(app: FastAPI):
    async with engine.begin() as conn:
        await conn.run_sync(Base.metadata.create_all)
    yield

app = FastAPI(lifespan=lifespan) # type: ignore


@app.get("/items/")
async def read_items(q: Annotated[List[str] | None, Query()] = None):
    query_items = {"q": q}
    return query_items

Could you please try it without using Annotated, as demonstrated here? — Chris
– Chris, Commented Jan 17, 2024 at 7:53
@Chris I think using Annotated is preferred way now because of pydantic2.0. — sudden_appearance
– sudden_appearance, Commented Jan 17, 2024 at 8:02
Annotated[List[str], Query()] = None does the thing. But it looks incorrect:') — sudden_appearance
– sudden_appearance, Commented Jan 17, 2024 at 8:07
async def read_items(q: list[str] | None = Query(default=None)): tried but didn't work — moth
– moth, Commented Jan 17, 2024 at 8:11
worked for me yes: Annotated[list[str],Query()] = None): but pylance complains about typing — moth
– moth, Commented Jan 17, 2024 at 8:14

tread · Accepted Answer · 2024-06-21 06:53:21Z

2

It seems to be the FastAPI example:

@app.get("/items/")
async def read_items(q: Annotated[list[str] | None, Query()] = None):
    query_items = {"q": q}
    return query_items

gives:

TypeError: unsupported operand type(s) for |: 'types.GenericAlias' and 'NoneType'

Removing the | None makes it work:

@app.get("/items/")
async def read_items(q: Annotated[list[str], Query()] = None):
    query_items = {"q": q}
    return query_items

Above is with:

fastapi 0.110.0
python 3.12

edited Jun 21, 2024 at 6:53

answered Jun 19, 2024 at 10:01

tread

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1 Comment

linello Over a year ago

How about when the query is from a pydantic object whose fields are lists of strings? I don't get the swagger rendering for them, only for scalar fields.

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