关于嵌套函数运行代码出错

Question

```python


class solution:
    def sequ(self,low:int,high:int):

        setA = self.helper(high)
        setB = self.helper(low-1)
        for num in setB:
            if num in setA:
                setA.remove(num)
        return sorted(list(setA))

    def helper(self,x):
        mySet = set()
        def dfs(cur_num,next_digit):
            nonlocal x,mySet
            if(cur_num>x):
                return
            else:
                mySet.add(cur_num)
                if(next_digit == 10):
                    return
                dfs(cur_num*10 + next_digit,next_digit + 1)

            for i in range(1,10):
                dfs(0,i)
            return mySet

if __name__ == '__main__':
    li = solution()
    li.sequ(200,2000)

这是我调用的代码，但是无法运行，报错内容如下：
Traceback (most recent call last):
  File "D:/Program Files (x86)/anaconda/envs/learn tensorflow/tushare1.py", line 734, in <module>
    li.sequ(200,2000)
  File "D:/Program Files (x86)/anaconda/envs/learn tensorflow/tushare1.py", line 709, in sequ
    setA = set(self.helper(high))
TypeError: 'NoneType' object is not iterable

1，是不是setA的设置出错了。
2， helper函数是不是分别被high，和low调用了两次。

Answer 1

你的helper函数实际上只执行了：

mySet = set()
return None

这两句，dfs根本没有被调用