最近学python，看老师写的代码看呆了 
def countdown(k):
    if k > 0:
        yield k
        yield from countdown(k - 1)
    else:
        yield 'Blast off'

def prefixes(s):
    if s:
        yield from prefixes(s[:-1])
        yield s

def substrings(s):
    if s:
        yield from prefixes(s)
        yield from substrings(s[1:])


In [2]: (list(countdown(5)),list(prefixes('both')),list(substrings('tops')))
O...