def evens(start, end):
    even = start + (start%2)
    while even<end:
        yield even
        even+=2

if __name__ == "__main__":
    t = evens(2,10)
    for i in t:
        print(i)

def countdown(k):
    if k>0:
        yield k
        yield from countdown(k-1)
        '''
        等价于 
        for i in countdown(k-1):
            yield i
        '''
    else:
        yield 'Blast off'
if __name__ == "__main__":
    t = countdown(3)
    for i in t:
        print(i)

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:])
if __name__ == "__main__":
    print(list(prefixes("both")))
    print(list(substrings("top")))
    

def partitions(n,m):
    if n<0 or m==0:
        return []
    else:
        exact_match = []
        if n==m:
            exact_match = [str(m)]
        with_m = [p+'+'+str(m) for p in partitions(n-m,m)]
        without_m = partitions(n,m-1)
        return exact_match+with_m+without_m
if __name__ == "__main__":
    for p in partitions(6,4):
        print(p)

def partitions(n,m):
    if n>0 and m>0:
        if n==m:
            yield str(m)
        for p in partitions(n-m,m):
            yield p+'+'+str(m)
        yield from partitions(n,m-1)
if __name__ == "__main__":
    t = partitions(6,4)
    for p in range(5):
        print(next(t))