https://docs.python.org/3/library/asyncio-stream.html?highlight=streamreaderprotocol#stream-functions says:

> (If you want to customize the StreamReader and/or StreamReaderProtocol classes, just copy the code – there’s really nothing special here except some convenience.)

StreamReaderProtocol inherits from streams.FlowControlMixin which isn't documented. Applying this advice means, instead of inheriting from StreamReaderProtocol, inheriting from streams.FlowControlMixin -- in order to obtain the _drain_helper method, which StreamWriter.drain must wait for.

At this point inheriting StreamReaderProtocol appears to be the lesser evil.

I suggest to remove this paragraph from the documentation.