Copy from: https://docs.python.org/3/library/asyncio.html

Today I'm gonna talk about how to programming with asyncio TCP connection. For general, you won't have much problems with server end. So I'm gonna talk about client.

import asyncio

class EchoClientProtocol(asyncio.Protocol):
    def __init__(self, message, loop):
        self.message = message
        self.loop = loop

    def connection_made(self, transport):
        transport.write(self.message.encode())
        print('Data sent: {!r}'.format(self.message))

    def data_received(self, data):
        print('Data received: {!r}'.format(data.decode()))

    def connection_lost(self, exc):
        print('The server closed the connection')
        print('Stop the event loop')
        self.loop.stop()

loop = asyncio.get_event_loop()
message = 'Hello World!'
coro = loop.create_connection(lambda: EchoClientProtocol(message, loop),
                              '127.0.0.1', 8888)
loop.run_until_complete(coro)
loop.run_forever()
loop.close()

If we just look at this example, you'll find a lot of interesting things that asyncio already did for us.

Before we get started, I'd like to tell you some fundamental data types that you should know first: protocol and transport.

https://docs.python.org/3/library/asyncio-protocol.html#protocols
https://docs.python.org/3/library/asyncio-protocol.html#basetransport

protocol, as we can see, just a class definition that defines what we should do when we connected to a server.
transport is a higher abstraction and encapsulation of socket, which allows us forget how socket do data exchanging. So we can use it sending data directly.

Firstly, we got connection_made callback function. This function allows us do something when connection has been created.

From this function, we'll get a transport instance class, that allows us sending message to server in later. So normally we wanna keep it as a global variable.

Then we got data_received callback function, which get data from server when message comes.

So if everything just OK, I'd rather make a reply client in this way:

import asyncio

class EchoClientProtocol(asyncio.Protocol):
    def __init__(self, message, loop):
        self.message = message
        self.loop = loop

    def connection_made(self, transport):
        transport.write(self.message.encode())
        print('Data sent: {!r}'.format(self.message))

        self.transport = transport

    def data_received(self, data):
        print('Data received: {!r}'.format(data.decode()))
        
        self.transport.write(data)

    def connection_lost(self, exc):
        print('The server closed the connection')
        print('Stop the event loop')
        self.loop.stop()

loop = asyncio.get_event_loop()
message = 'Hello World!'
coro = loop.create_connection(lambda: EchoClientProtocol(message, loop),
                              '127.0.0.1', 8888)
loop.run_until_complete(coro)
loop.run_forever()
loop.close()