Headers and MutableHeaders incorrectly assumes type of scope["headers"]

[jarojasm95]

Looking at the asgi docs - https://asgi.readthedocs.io/en/latest/specs/www.html#http-connection-scope

It specifies:

• headers (Iterable[[byte string, byte string]])

However Starlette Headers assumes it is a list - https://github.com/encode/starlette/blob/master/starlette/datastructures.py#L525 which causes MutableHeaders to break when adding a new header (because it wants to use list.append() https://github.com/encode/starlette/blob/master/starlette/datastructures.py#L608)

I've found that nginx unit (https://unit.nginx.org/) passes a tuple instead of a list, surfacing this issue.

[alex-oleshkevich]

Headers and MutableHeaders serve different purposes. Commonly, Headers is a read only struct that used by request.headers (scope['headers']) and not intended to be modifiable.
MutableHeaders is for responses where you can dynamically alter headers.

The ASGI spec tells that headers are Iterable object (used in for-in like constructs). Starlette conforms this protocol.

If you want to modify headers in middleware, I suggest converting scope['headers'] to MutableHeaders first.

[alex-oleshkevich]

If case of Headers the type of _list is irrelevant. All we need to know is that we can iterate it.

[adriangb]

We could make this more explicit with a better variable name and type

[adriangb]

Also we should cast it to a list in mutableheaders

[alex-oleshkevich]

Yes, it makes sense to convert variable to List at this line

starlette/starlette/datastructures.py

Line 525 in 858629f

self._list = scope["headers"]

[adriangb]

How does #1917 look?