import asyncio


class EchoServerProtocol(asyncio.Protocol):
    def connection_made(self, transport):
        self.transport = transport

    def data_received(self, data):
        self.transport.write(data)
        self.transport.close()


async def main():
    # Get a reference to the event loop as we plan to use
    # low-level APIs.
    loop = asyncio.get_running_loop()

    server = await loop.create_server(
        lambda: EchoServerProtocol(),
        host="",
        port=0,
    )

    ports = set()
    for socket in server.sockets:
        host, port = socket.getsockname()[:2]
        print(f"listening on {host}:{port}")
        ports.add(port)

    assert len(ports) == 1, "Only 1 port expected!"

    async with server:
        await server.serve_forever()


asyncio.run(main())