实现自己的xrange

def myrange(start, end=None, step=1):
    if end == None:
        end = start
        start = 0
    if step > 0:
        while start < end:
            yield start
            start += step
    elif step < 0:
        while start > end:
            yield start
            start += step
    else:
        return 'step can not be zero'

for i in myrange(12, 10, -1):
    print(i)