# generator.py

def countdown_task(n):
    while n > 0:
        print n
        yield
        n -= 1

# A list of tasks to run
from collections import deque
tasks = deque([
    countdown_task(5),
    countdown_task(10),
    countdown_task(15)
])

def scheduler(tasks):
    while tasks:
        task = tasks.popleft()
        try:
            next(task)         # Run to the next yield
            tasks.append(task) # Reschedule
        except StopIteration:
            pass

# Run it
scheduler(tasks)