Python asyncio port scanner

This is a quick python port scanner using asyncio module.

Key points:

• It doesn't use multiprocessing or multithreading; it uses asyncronous processes. The scripts sends the requests one after another and processes the replies when they arrive.
• I used semaphore to limit the request counts. It seems there's a limitation on both Windows and Linux machines. source
• Adding timeout is important because some connection attempts may hang. asyncio.wait_for is used for that purpose.

Have fun!

import asyncio

async def check_port(ip, port, loop):
    conn = asyncio.open_connection(ip, port, loop=loop)
    try:
        reader, writer = await asyncio.wait_for(conn, timeout=3)
        print(ip, port, 'ok')
        return (ip, port, True)
    except:
        print(ip, port, 'nok')
        return (ip, port, False)
    finally:
        if 'writer' in locals():
            writer.close()

async def check_port_sem(sem, ip, port, loop):
    async with sem:
        return await check_port(ip, port, loop)

async def run(dests, ports, loop):
    sem = asyncio.Semaphore(400) #Change this value for concurrency limitation
    tasks = [asyncio.ensure_future(check_port_sem(sem, d, p, loop)) for d in dests for p in ports]
    responses = await asyncio.gather(*tasks)
    return responses

dests = ['steemit.com', 'steem.io', 'www.raiblocks.net', 'bitcoin.org'] #destinations
ports = [80, 443, 8080, 8443] #ports

loop = asyncio.get_event_loop()
future = asyncio.ensure_future(run(dests, ports, loop))
loop.run_until_complete(future)
print('#'*50)
print('Results: ', future.result())

Output:

steem.io 443 ok
steem.io 80 ok
www.raiblocks.net 443 ok
www.raiblocks.net 80 ok
www.raiblocks.net 8443 ok
www.raiblocks.net 8080 ok
steemit.com 80 ok
steemit.com 443 ok
bitcoin.org 80 ok
bitcoin.org 443 ok
steemit.com 8080 nok
steem.io 8080 nok
bitcoin.org 8080 nok
bitcoin.org 8443 nok
steem.io 8443 nok
steemit.com 8443 nok
##################################################
Results:  [('steemit.com', 80, True), ('steemit.com', 443, True), ('steemit.com', 8080, False), ('steemit.com', 8443, False), ('steem.io', 80, True), ('steem.io', 443, True), ('steem.io', 8080, False), ('steem.io', 8443, False), ('www.raiblocks.net', 80, True), ('www.raiblocks.net', 443, True), ('www.raiblocks.net', 8080, True), ('www.raiblocks.net', 8443, True), ('bitcoin.org', 80, True), ('bitcoin.org', 443, True), ('bitcoin.org', 8080, False), ('bitcoin.org', 8443, False)]