TypeError: 'NoneType' object is not iterable given when using 'sum()' on a list

Question

I am getting

TypeError: 'NoneType' object is not iterable

when I try to find the sum of a list.

Where the problem occurs:

if(sum(self._candidates) + self._allCandidates[self._depth]._weight > 20):
    self._left = Node(self._candidates, self._depth + 1, self._allCandidates)
else:
    self._left = Node(self._candidates.append(self._allCandidates[self._depth]), self._depth + 1, self._allCandidates)

Node definition:

def __init__(self, candidates = [], depth = -1, allCandidates = []):
        self._candidates = candidates
        self._depth = depth
        self._allCandidates = allCandidates

Thanks for any help on this matter.

Answer 1

This is wrong:

Node(self._candidates.append(self._allCandidates[self._depth])

The return value from .append is None, hence the error.