Simple Python asyncio server and client with a JSON protocol

client.py

#!/usr/bin/env python3

import asyncio
import json
import sys


async def main():
    reader, writer = await asyncio.open_connection("127.0.0.1", 8888)
    messages = ["One", 2, {"thr": "ee"}, ["4"], [5, 6]]
    for message in messages:
        print(f"sending: {message}")
        payload = json.dumps(message).encode()
        size = len(payload)
        writer.write(size.to_bytes(4, byteorder="big"))
        writer.write(payload)
        _, response = await asyncio.gather(writer.drain(), reader.read(4))
        if len(response) != 4:
            raise Exception(
                f"incomplete response; expected 4 bytes, got {len(response)}")
        response_size = int.from_bytes(response, byteorder="big")
        response = await reader.read(response_size)
        j = json.loads(response)
        print(f"response: {j}")


if __name__ == "__main__":
    sys.exit(asyncio.run(main()))

server.py

#!/usr/bin/env python3

import asyncio
import json
import sys


async def main():
    server = await asyncio.start_server(handle_outer_connection, "127.0.0.1",
                                        8888)
    addrs = [s.getsockname() for s in server.sockets]
    print(f"listening on {addrs}")
    async with server:
        await server.serve_forever()
    return 0


async def handle_outer_connection(reader, writer):
    print("got connection")
    try:
        await handle_connection(reader, writer)
    except Exception as e:
        print(f"caught exception: {e}")
    finally:
        print("closing connection")
        writer.close()
        await writer.wait_closed()


async def handle_connection(reader, writer):
    while True:
        req_size = await reader.read(4)
        if len(req_size) == 0 and reader.at_eof():
            return
        elif len(req_size) != 4:
            raise Exception(f"incomplete request size: {len(req_size)}")
        req_size = int.from_bytes(req_size, byteorder="big")
        request = await reader.read(req_size)
        request_j = json.loads(request)
        response = {"response": request_j}
        response_j = json.dumps(response).encode()
        response_size = len(response_j)
        writer.write(response_size.to_bytes(4, byteorder="big"))
        writer.write(response_j)
        await writer.drain()


if __name__ == "__main__":
    sys.exit(asyncio.run(main()))