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def count_to(n):
    for i in range(n):
        yield i

for i in count_to(5):
    print(i)


t = count_to(5) #   t is like a generator object at <generator object count_to at 0x000001>
print(next(t))

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def countdown(k):
    if k > 0:
        yield k
        yield from countdown(k-1) # element-wise iteration of the sub-generator

for i in countdown(5):
    print(i)

list(countdown(5)) # [5, 4, 3, 2, 1]