The fix for issue #2364 ("AbstractServer" has no attribute "sockets") added this declaration:

class AbstractServer:
    sockets: Optional[List[socket]]

However, in the Python 3.9 implementation of asyncio.base_events.Server, sockets is a property defined like this:

    @property
    def sockets(self):
        if self._sockets is None:
             return ()
        return tuple(trsock.TransportSocket(s) for s in self._sockets)

So we have two problems:

1. The value of sockets is never None, it's always a tuple.
2. It is really a tuple[TransportSocket, ...].

There is example code in the asyncio docs and test code in the standard library that assume sockets is a sequence of sockets.

@ymyzk contributed the patch for #2364 noting "I noticed that this class has some changes in Python 3.7, so we need to update it again in the near future."

The documentation for asyncio.Server.sockets states:

Changed in version 3.7: Prior to Python 3.7 Server.sockets used to return an internal list of server sockets directly. In 3.7 a copy of that list is returned.

As I mentioned, the actual value returned is a tuple of zero or more asyncio.Server.TransportSocket objects.