Asynchronous Function Call

Question

I would like to learn how to call a function asynchronously in Python3. I think Tornado can do this. Currently, my code is returning nothing on the command line:

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

async def count(end):
    """Print message when start equals end."""
    start = 0
    while True:
        if start == end:
            print('start = {0}, end = {1}'.format(start, end))
            break
        start = start + 1

def main():

    # Start counting.
    yield count(1000000000)

    # This should print while count is running.
    print('Count is running. Async!')

if __name__ == '__main__':
    main()

Thanks

Answer 1

To call an async function, you need to provide an event loop to handle it. If you have a Tornado app, it provides such a loop, which allows you to make your handlers asynchronous:

from tornado.web import RequestHandler, url
from tornado.httpserver import HTTPServer
from tornado.ioloop import IOLoop


async def do_something_asynchronous():
    # e.g. call another service, read from database etc
    return {'something': 'something'}


class YourAsyncHandler(RequestHandler):

    async def get(self):
        payload = await do_something_asynchronous()
        self.write(payload)


application = web.Application([
    url(r'/your_url', YourAsyncHandler, name='your_url')
])

http_server = HTTPServer(application)
http_server.listen(8000, address='0.0.0.0')
IOLoop.instance().start()

Outside of a Tornado app you can get the event loop from any number of providers, including the built-in asyncio library:

import asyncio
event_loop = asyncio.get_event_loop()
try:
    event_loop.run_until_complete(do_something_asynchronous())
finally:
    event_loop.close()