Multitasking example using coroutines

multitasking.py

import itertools
import collections


def run(tasks):
    # Prepare
    results = {}
    count = itertools.count()
    queue = collections.deque()
    for task in tasks:
        queue.append((next(count), task))
    # Run
    while queue:
        # Get next task
        _, task = queue.popleft()
        # Run steps
        try:
            next(task)
        # End of coroutine
        except StopIteration as exc:
            results[task] = exc.value
        # Exception
        except Exception as exc:
            results[task] = exc
        # Schedule next step
        else:
            queue.append((next(count), task))
    # Return results
    return [results[task] for task in tasks]


def a():
    yield
    return 2


def b():
    yield
    yield
    return 3


def x():
    ra = yield from a()
    rb = yield from b()
    return ra + rb


def y():
    yield
    raise RuntimeError('Oops')


print(run([x(), y()]))  # [5, RuntimeError('Oops',)]