Uvicorn assumes that HTTPResponseStartEvent.headers is a List

[rijenkii]

According to the spec and asgiref, HTTPResponseStartEvent.headers is Iterable[Tuple[bytes, bytes]].

But, if http.response.start is sent like this:

headers: dict
await send({
  "type": "http.response.start",
  "status": status,
  "headers": headers.items(),
})

Uvicorn fails with

  File ".../uvicorn/protocols/http/h11_impl.py", line 474, in send
    headers = self.default_headers + message_headers
              ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
TypeError: can only concatenate list (not "dict_items") to list

Relevant Uvicorn code:

uvicorn/uvicorn/protocols/http/h11_impl.py

Lines 471 to 474 in 782f17e

	message_headers = cast(
	List[Tuple[bytes, bytes]], message.get("headers", [])
	)
	headers = self.default_headers + message_headers

---

Potential fix may just be this:

message_headers = list(message.get("headers", [])) 

[Kludex]

PR welcome.

[Kludex]

This was fixed on #1782, and it will be available on the next uvicorn release (current version is 0.20.0).