Asynchronous map for Tornado's ioloop

async_map.py

from tornado.ioloop import IOLoop

# Calls fn for all items of iterable, saves result to a list, and
# calls callback with that list. This is most useful when fn works
# asynchronously.
def async_map(fn, iterable, callback, io_loop=None):
    ioloop = io_loop or IOLoop.instance()

    def loop():
        try:
            arg = next(iterator)
        except StopIteration:
            callback(result)
        else:
            fn(arg, save_result)

    def save_result(value):
        result.append(value)
        ioloop.add_callback(loop)

    iterator = iter(iterable)
    result = []

    ioloop.add_callback(loop)

example.py

import functools
import tornado.ioloop
from async_map import async_map

ioloop = tornado.ioloop.IOLoop.instance()

def start():
    async_map(do_something, [1, 2, 3, 4], done)

def do_something(i, callback):
    # A simple example of doing something asynchronously.
    ioloop.add_callback(functools.partial(callback, i + 1))

def done(results):
    assert results == [2, 3, 4, 5]
    ioloop.stop()

start()
ioloop.start()