The type annotation for Session.peer is currently:

self.peer: Optional[str] = None

This attribute appears to be set in connection_made() like so:

self.session.peer = transport.get_extra_info("peername")

Where transport is an instance of asyncio.BaseTransport.

Looking at BaseTransport docs, get_extra_info("peername") calls socket.getpeername() and returns the result.

For IPv4 sockets at least, socket.getpeername() seems to return an ip-address,port tuple. So I think a more accurate annotation for Session.peer would be something like tuple[str, int] | None.