FastAPI response_model unable to handle Iterable types -- returns only the model keys

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Example Code

from typing import Any, Iterable, List
from fastapi import FastAPI, APIRouter
from pydantic import BaseModel

# Define simple model
class Simple(BaseModel):
    spam: int
    eggs: str

# instantiate fast api and route
loader = FastAPI(
    version="0",
)

requests_router = APIRouter(prefix="/requests", tags=["requests"])

@requests_router.get("/", response_model=Iterable[Simple])
def get_requests() -> Iterable[Simple]:
    return [
        Simple(
            spam=1,
            eggs="a"
        )
    ]

loader.include_router(requests_router)

###
### Output at http://127.0.0.1:8000/requests/ is 
### {
###     "spam": "eggs"
### }
###

Description

To reproduce

• Run uvicorn api2:loader --reload
• Go to http://127.0.0.1:8000/requests/
• Observe only the keys in the data model are returned

To fix

• Change the response_model to response_model=List[Simple]
• Go to http://127.0.0.1:8000/requests/
• Observe you get the correct response:

[
    {
        "spam": 1,
        "eggs": "a"
    }
]

I haven’t been able to find this in any documentation or bugs, but maybe my google skills are lacking

uvicorn            0.15.0
fastapi            0.68.1

Operating System

Windows

Operating System Details

Windows 11 21H2 (OS Build 22000.160))

FastAPI Version

0.68.1

Python Version

3.9.1

Additional Context

This has a workaround, but I don’t know the code base well enough to understand why an Iterable type would cause this problem

[roj1512]

Need this

[klaa97]

The main issue is that the jsonable_encoder doesn’t support raw Iterator (produced by Pydantic when validating using as a response_model Iterable[Simple]), but it does support List .

I opened #3913 to try to fix the behavior - I tested in local your example code and it should work as you expect.