自己用python的yield语句实现了一个递归生成全排列的生成器，不可用，产生StopIterator异常，请高手指教？

Question

用递归实现的全排列生成器如下：

num = [str(i) for i in xrange(10)]
def perm(num,s):
    if s==len(num):
        yield ''.join(num)
    for i in xrange(s,len(num)):
        num[s],num[i] = num[i],num[s]
        perm(num,s+1)
        num[s],num[i] = num[i],num[s]
f=perm(num,0)
print f.next()

运行后提示如下：

Traceback (most recent call last):
    File "###",line 10,in <module>
        f.next()
StopIteration

我自己检查，发现递归根本进不去啊，是不是我使用yield的方法不对啊?

Answer 1

def perm(num,s):
    if s==len(num):
        yield ''.join(num)
    for i in xrange(s,len(num)):
        num[s],num[i] = num[i],num[s]
        for each in perm(num,s+1):
            yield each
        num[s],num[i] = num[i],num[s]