[FIXED] Python simple socket client/server using asyncio

Issue

I would like to re-implement my code using asyncio coroutines instead of multi-threading.

server.py

def handle_client(client):
    request = None
    while request != 'quit':
        request = client.recv(255).decode('utf8')
        response = cmd.run(request)
        client.send(response.encode('utf8'))
    client.close()

server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.bind(('localhost', 15555))
server.listen(8)

try:
    while True:
        client, _ = server.accept()
        threading.Thread(target=handle_client, args=(client,)).start()
except KeyboardInterrupt:
    server.close()

client.py

server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.connect(('localhost', 15555))
request = None

try:
    while request != 'quit':
        request = input('>> ')
        if request:
            server.send(request.encode('utf8'))
            response = server.recv(255).decode('utf8')
            print(response)
except KeyboardInterrupt:
    server.close()

I know there are some appropriate asynchronous network librairies to do that. But I just want to only use asyncio core library on this case in order to have a better understanding of it.

It would have been so nice to only add async keyword before handle client definition... Here a piece of code which seems to work, but I'm still confused about the implementation.

asyncio_server.py

def handle_client(client):
    request = None
    while request != 'quit':
        request = client.recv(255).decode('utf8')
        response = cmd.run(request)
        client.send(response.encode('utf8'))
    client.close()

def run_server(server):
    client, _ = server.accept()
    handle_client(client)

server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.bind(('localhost', 15555))
server.listen(8)

loop = asyncio.get_event_loop()
asyncio.async(run_server(server))
try:
    loop.run_forever()
except KeyboardInterrupt:
    server.close()

How adapt this in the best way and using async await keywords.

Solution

The closest literal translation of the threading code would create the socket as before, make it non-blocking, and use asyncio low-level socket operations to implement the server. Here is an example, sticking to the more relevant server part (the client is single-threaded and likely fine as-is):

import asyncio, socket

async def handle_client(client):
    loop = asyncio.get_event_loop()
    request = None
    while request != 'quit':
        request = (await loop.sock_recv(client, 255)).decode('utf8')
        response = str(eval(request)) + '\n'
        await loop.sock_sendall(client, response.encode('utf8'))
    client.close()

async def run_server():
    server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    server.bind(('localhost', 15555))
    server.listen(8)
    server.setblocking(False)

    loop = asyncio.get_event_loop()

    while True:
        client, _ = await loop.sock_accept(server)
        loop.create_task(handle_client(client))

asyncio.run(run_server())

The above works, but is not the intended way to use asyncio. It is very low-level and therefore error-prone, requiring you to remember to set the appropriate flags on the socket. Also, there is no buffering, so something as simple as reading a line from the client becomes a tiresome chore. This API level is really only intended for implementors of alternative event loops, which would provide their implementation of sock_recv, sock_sendall, etc.

Asyncio's public API provides two abstraction layers intended for consumption: the older transport/protocol layer modeled after Twisted, and the newer streams layer. In new code, you almost certainly want to use the streams API, i.e. call asyncio.start_server and avoid raw sockets. That significantly reduces the line count:

import asyncio, socket

async def handle_client(reader, writer):
    request = None
    while request != 'quit':
        request = (await reader.read(255)).decode('utf8')
        response = str(eval(request)) + '\n'
        writer.write(response.encode('utf8'))
        await writer.drain()
    writer.close()

async def run_server():
    server = await asyncio.start_server(handle_client, 'localhost', 15555)
    async with server:
        await server.serve_forever()

asyncio.run(run_server())

Answered By - user4815162342