# multitask.py

from collections import deque

tasks = deque()
def run():
    while tasks:
        task = tasks.popleft()
        try:
            next(task)
            tasks.append(task)
        except StopIteration:
            print('Task done')

def countdown(n):
    while n > 0:
        print('T-minus', n)
        yield
        n -= 1

def countup(n):
    x = 0
    while x < n:
        print('Up we go', x)
        yield
        x += 1

if __name__ == '__main__':
    tasks.append(countdown(10))
    tasks.append(countdown(5))
    tasks.append(countup(20))
    run()