TypeError：'NoneType'对象不可迭代:(

Question

What i am trying to do is making a powerset by given set. nums. But i am stuck in a problem right now. I can not understand why python just saying NoneType error...

+Q. I am wondering if list_1 could interact with local list. In my recursion func, i want to discribe 3th argument as list_ but doing like that occurs other problems. maybe process with list_ dont apply to real list list_1.

def powerset(nums):
    list_1=[]
    recursion(0,nums,list_1)
    print (list_1)

def recursion(start,nums,list_1):
    if start>len(nums)-1:
        list_1.append([])
        return 0
    recursion(start+1,nums,list_1)
    i=0
    save_list=list_1
    save_len=len(list_1)
    while i<save_len:
        list_1.extend(save_list[i].append(nums[start]) )
        i += 1

Answer 1

def powerset(nums):
    list_1=[]
    recursion(0,nums,list_1)
    print (list_1)

def recursion(start,nums,list_):
    if start>len(nums)-1:
        list_.append([])
        return 0
    recursion(start+1,nums,list_)
    i=0
    save_list=list_
    save_len=len(list_)
    while i<save_len:
        listy=save_list[i]
        listy.append(nums[start])
        list_[i].extend(listy)
        i += 1

Try this. Your problem was with append as chepner said.