asyncio.StreamWriter wraps a transport. The first three document methods of asyncio.BaseTransport are close(), is_closing() and get_extra_info().

It is somewhat surprising that StreamWriter provides close() and get_extra_info() but not is_closing(). I'm proposing that StreamWriter implement is_closing() as well.

It's as simple as:

    def is_closing(self):
        return self._transport.is_closing()

Perhaps it was simply missed in https://github.com/python/asyncio/pull/291.

It's trivial to work around this omission with stream_writer.transport.is_closing().

I'm only suggesting to add it for consistency.