Demonstrates a minor difference between the asyncio and uvloop create_server handling of the port argument.

create_server.py

#!/usr/bin/env python3.5

'''
This script can be used to demonstrate a minor difference
between acceptable port argument passed to the `create_server`
method on the `asyncio` and `uvloop` event loop.

When the script is run without any args it uses `asyncio` to
create a server socket using some candidate `port` tokens that
instruct the event loop to create a server on an ephemeral port.

..code-block:: console

    $ create_server.py
    Creating server using loop:asyncio, port_candidate:0
    Server was bound on port: 51862
    Creating server using loop:asyncio, port_candidate:None
    Server was bound on port: 51863

When the script is run with the `--uvloop` arg it uses `uvloop`
to run the same checks.

..code-block:: console

    $ create_server.py --uvloop
    Creating server using loop:uvloop, port_candidate:0
    Server was bound on port: 55882
    Creating server using loop:uvloop, port_candidate:None
    Failed to bind server using loop:uvloop, port_candidate:None

'''

import argparse
import asyncio
import socket


async def main(loop, loop_name):
    '''
    Bind a server to an ephemeral port using a range of
    port candidate tokens.
    '''

    port_candidates = [0, None]

    for port_candidate in port_candidates:
        try:
            # create server
            print(
                'Creating server using loop:{}, port_candidate:{}'.format(
                    loop_name, port_candidate))
            listener = await loop.create_server(
                asyncio.Protocol,
                '127.0.0.1',
                port_candidate,
                family=socket.AF_INET)

            # fetch details
            host, port = listener.sockets[0].getsockname()
            print('Server was bound on port:', port)

            # close server
            listener.close()
            await listener.wait_closed()

        except:
            print(
                'Failed to bind server using loop:{}, '
                'port_candidate:{}'.format(
                    loop_name, port_candidate))


ARGS = argparse.ArgumentParser(description='Create server tester')
ARGS.add_argument(
    '--uvloop', action="store_true",
    help="Use the uvloop instead of default asyncio event loop")


if __name__ == '__main__':

    args = ARGS.parse_args()

    loop_name = 'asyncio'
    if args.uvloop:

        import uvloop
        asyncio.DefaultEventLoopPolicy = uvloop.EventLoopPolicy
        asyncio.set_event_loop_policy(uvloop.EventLoopPolicy())
        loop_name = 'uvloop'

    loop = asyncio.get_event_loop()

    try:
        loop.run_until_complete(main(loop, loop_name))
    except KeyboardInterrupt:
        pass
    finally:
        loop.stop()
        loop.close()