Note that Glyph recommends against this fix (see mailing list).

On Thu, Jun 11, 2015 at 7:59 AM, Martin Teichmann <report@bugs.python.org>
wrote:

>
> New submission from Martin Teichmann:
>
> Currently there is an assert statement asserting that no two
> tasks (asyncio tasks, that is) can use StreamWriter.drain at
> the same time. This is a weird limitiation, if there are two
> tasks writing to the same network socket, there is no reason
> why not both of them should drain the socket after (or before)
> writing to it.
>
> A simple bug fix is attached.
>
> ----------
> components: asyncio
> files: patch
> messages: 245172
> nosy: Martin.Teichmann, gvanrossum, haypo, yselivanov
> priority: normal
> severity: normal
> status: open
> title: StreamWriter.drain is not callable concurrently
> type: behavior
> versions: Python 3.4, Python 3.5, Python 3.6
> Added file: http://bugs.python.org/file39681/patch
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <http://bugs.python.org/issue24431>
> _______________________________________
>