Eventually planned to be a decent netcat replacement for windows.

pync.py

import asyncio
import sys
import socket

import signal
signal.signal(signal.SIGINT, signal.SIG_DFL)

from argparse import ArgumentParser

def parse_cl():
    parser = ArgumentParser(description='Python netcat')
    parser.add_argument('-l', '--listen', action='store_true',
                        help='Listen')
    parser.add_argument('host',
                        help='Host')
    parser.add_argument('port', type=int,
                        help='Port number')
    return parser

server = None

async def connection_handler(reader, writer):
    await connectionLoop(reader, writer)
    server.close()

async def create_client(host, port):
    reader, writer = await asyncio.open_connection(
        host, port, family=socket.AF_INET6)
    
    await connectionLoop(reader, writer)

async def listen_server(port):
    global server
    server = await asyncio.start_server(
        connection_handler, None, port, family=socket.AF_INET6)

    addr = server.sockets[0].getsockname()
    print(f'Serving on {addr}')

    async with server:
        await server.wait_closed()
        print("Server closed")

async def connectionLoop(reader, writer):
    write_task = asyncio.create_task(pushLoop(writer))
    await pullLoop(reader)
    # Cancel writing, connection is closing.
    write_task.cancel()
    writer.close()

    print("Connection cleaned up.")

def get_line_from_stdin():
    return sys.stdin.readline()

async def asyncReadline():
    return await asyncio.get_running_loop().run_in_executor(None, get_line_from_stdin)

async def pushLoop(writer):
    while True:
        line = await asyncReadline()
        if line == '' or line == None:
            break
        print("writing line: ", line)
        writer.write(line.encode())
        await writer.drain()
    print("Finished writing.")
    writer.close()

async def pullLoop(reader):
    while True:
        line = await reader.readline()
        if len(line) <= 0:
            break
        print("Got: ", line.decode())
    print("Finished reading from remote.")

if __name__ == '__main__':
    parser = parse_cl()
    args = parser.parse_args()

    if args.listen:
        asyncio.run(listen_server(args.port))
    else:
        asyncio.run(create_client(args.host, args.port))