Python中迭代器为空时如何获取None

Question

I implemented a generator in Python, the problem is that I want to return "None" when the generator is empty:

def permute(items):
    permutations = [x for x in itertools.permutations(items)]
    permutations.sort()
    it = iter(permutations)
    while True:
        try:
            new_items = yield next(it)
            if new_items is not None:
                permutations = [x for x in itertools.permutations(new_items)]
                permutations.sort()
                it = iter(permutations)
        except StopIteration:
            return None
print(next(g))
print(next(g))
print(next(g))
print(next(g))
print(next(g))
print(next(g))
print(next(g)) 

I get an error:

StopIteration

Answer 1

Your problem is the next(iter, None) . You loop never ends. It works correctly for 6 times, and then, you start yielding None . That when you get the error.

Remove the None , so that you cause a StopIteration and cause the outer loop to end

Answer 2

list(permute(['b', 'a', 'c']))

will return a list of the output of the generator.

However, your generator seem to return an infinite loop.