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.

I just re-read the docs here (which came straight from the open_connection() docstring). The parenthetical remark is confusing -- I meant something more like

"""
If you want to use different classes than StreamReader, StreamReaderProtocol and StreamWriter, just copy the code of open_connection() and substitute your own classes.
"""

That is, the customization it is trying to refer to is open_connection(), not the classes.

FWIW I think subclassing implementation classes is a dangerous practice and shouldn't be encouraged.

Because the same reasoning applies to all the top-level functions here (but not to the classes), I think I'll delete the offending paragraph and replace it with something like the following at the top of the docs for this module:

"""
The top-level functions in this module are meant convenience wrappers only; there's really nothing special there, and if they don't do what you want feel free to copy their code.
"""

New changeset d91208957e4e by Guido van Rossum in branch '3.4':
Issue #24885: Update note in docs about stream convenience functions.
https://hg.python.org/cpython/rev/d91208957e4e

New changeset 43c63dd2f383 by Guido van Rossum in branch '3.5':
Issue #24885: Update note in docs about stream convenience functions. (Merge 3.4->3.5)
https://hg.python.org/cpython/rev/43c63dd2f383

New changeset 7947f43a1a87 by Guido van Rossum in branch 'default':
Issue #24885: Update note in docs about stream convenience functions. (Merge 3.5->3.6)
https://hg.python.org/cpython/rev/7947f43a1a87