h11._util.LocalProtocolError: Too much data for declared Content-Length - when returning dict as response, with custom status code

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Example

Here’s a self-contained, minimal, reproducible, example with my use case:

from fastapi import FastAPI
from fastapi.responses import JSONResponse
from pydantic import BaseModel
import uvicorn

app = FastAPI()

class MyModel(BaseModel):
    foo: str
    bar: int

RESPONSE = {"message": "Hello there!"}

@app.get("/get1")
def get1():
    return RESPONSE

@app.get("/get2", status_code=204)
def get2():
    return RESPONSE

@app.post("/post1")
def post1():
    return RESPONSE

@app.post("/post2")
def post2(data: MyModel):
    return RESPONSE

@app.post("/post3", status_code=204)
def post3():
    return RESPONSE

@app.post("/post4", status_code=204)
def post4(data: MyModel):
    return RESPONSE

@app.post("/post5")
def post5(data: MyModel, status_code=204):
    return JSONResponse(
        content=RESPONSE,
        status_code=204 # must define status_code here
    )

if __name__ == "__main__":
    uvicorn.run(app, port=5000)

Description

On the given code, the endpoints /get2, /post3 and /post4 fail, raising this exception:

INFO:     127.0.0.1:46692 - "POST /post4 HTTP/1.1" 204 No Content
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "/home/david/.miniconda3/envs/fastapi+pydantic/lib/python3.8/site-packages/uvicorn/protocols/http/h11_impl.py", line 389, in run_asgi
    result = await app(self.scope, self.receive, self.send)
  File "/home/david/.miniconda3/envs/fastapi+pydantic/lib/python3.8/site-packages/uvicorn/middleware/proxy_headers.py", line 45, in __call__
    return await self.app(scope, receive, send)
  File "/home/david/.miniconda3/envs/fastapi+pydantic/lib/python3.8/site-packages/fastapi/applications.py", line 179, in __call__
    await super().__call__(scope, receive, send)
  File "/home/david/.miniconda3/envs/fastapi+pydantic/lib/python3.8/site-packages/starlette/applications.py", line 111, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/home/david/.miniconda3/envs/fastapi+pydantic/lib/python3.8/site-packages/starlette/middleware/errors.py", line 181, in __call__
    raise exc from None
  File "/home/david/.miniconda3/envs/fastapi+pydantic/lib/python3.8/site-packages/starlette/middleware/errors.py", line 159, in __call__
    await self.app(scope, receive, _send)
  File "/home/david/.miniconda3/envs/fastapi+pydantic/lib/python3.8/site-packages/starlette/exceptions.py", line 82, in __call__
    raise exc from None
  File "/home/david/.miniconda3/envs/fastapi+pydantic/lib/python3.8/site-packages/starlette/exceptions.py", line 71, in __call__
    await self.app(scope, receive, sender)
  File "/home/david/.miniconda3/envs/fastapi+pydantic/lib/python3.8/site-packages/starlette/routing.py", line 566, in __call__
    await route.handle(scope, receive, send)
  File "/home/david/.miniconda3/envs/fastapi+pydantic/lib/python3.8/site-packages/starlette/routing.py", line 227, in handle
    await self.app(scope, receive, send)
  File "/home/david/.miniconda3/envs/fastapi+pydantic/lib/python3.8/site-packages/starlette/routing.py", line 44, in app
    await response(scope, receive, send)
  File "/home/david/.miniconda3/envs/fastapi+pydantic/lib/python3.8/site-packages/starlette/responses.py", line 139, in __call__
    await send({"type": "http.response.body", "body": self.body})
  File "/home/david/.miniconda3/envs/fastapi+pydantic/lib/python3.8/site-packages/starlette/exceptions.py", line 68, in sender
    await send(message)
  File "/home/david/.miniconda3/envs/fastapi+pydantic/lib/python3.8/site-packages/starlette/middleware/errors.py", line 156, in _send
    await send(message)
  File "/home/david/.miniconda3/envs/fastapi+pydantic/lib/python3.8/site-packages/uvicorn/protocols/http/h11_impl.py", line 483, in send
    output = self.conn.send(event)
  File "/home/david/.miniconda3/envs/fastapi+pydantic/lib/python3.8/site-packages/h11/_connection.py", line 469, in send
    data_list = self.send_with_data_passthrough(event)
  File "/home/david/.miniconda3/envs/fastapi+pydantic/lib/python3.8/site-packages/h11/_connection.py", line 502, in send_with_data_passthrough
    writer(event, data_list.append)
  File "/home/david/.miniconda3/envs/fastapi+pydantic/lib/python3.8/site-packages/h11/_writers.py", line 78, in __call__
    self.send_data(event.data, write)
  File "/home/david/.miniconda3/envs/fastapi+pydantic/lib/python3.8/site-packages/h11/_writers.py", line 98, in send_data
    raise LocalProtocolError("Too much data for declared Content-Length")
h11._util.LocalProtocolError: Too much data for declared Content-Length

It seems that this fails when returning a dict as response (without using JSONResponse), AND setting a status_code for the endpoint.

Environment

• OS: Ubuntu 18.04
• Python version: 3.8.5
• FastAPI version: 0.61.1
• Uvicorn version: 0.12.2