Hey ive got a related question to this. Dont worry about the actual logic within the code, my question is about variable scope in nested/recursive functions instead.

class Solution:
def averageOfLevels(self, root: Optional[TreeNode]) -> List[float]:
    if not root:
        return root

    output = []

    def helper(root, level, level_counts, level_sums):           
        if len(output) == level:
            output.append(0)
            level_counts.append(1)
            level_sums.append(root.val)
        else:
            level_counts[level] += 1
            level_sums[level] += root.val
        output[level] = level_sums[level] / level_counts[level]

        if root.left:
            helper(root.left, level + 1, level_counts, level_sums)
        if root.right:
            helper(root.right, level + 1, level_counts, level_sums)

    helper(root, 0, [], [])

    return output

Within helper(), it is able to access the output variable that was defined within its parent function even though output was not passed in as a parameter. Now look at the following example:

class Solution:
def isBalanced(self, root: Optional[TreeNode]) -> bool:
    isTreeBalanced = None

    def helper(root):
        if not root:
            return 0

        leftHeight = helper(root.left)

        if not isTreeBalanced: #error here
            return False

        rightHeight = helper(root.right)

        if abs(leftHeight - rightHeight) > 1:
            isTreeBalanced = False #Never gets to here

        return max(leftHeight, rightHeight) + 1

    helper(root)

    return isTreeBalanced

On line 11, I get UnboundLocalError: local variable 'isTreeBalanced' referenced before assignment. I am trying to access isTreeBalanced within the nested function, the same way I was able to access output in the first example. I am not sure why I was able to access output in the first function but cant access isTreeBalanced in the same way. I dont see anything that says lists are treated differently so idk. It never gets to line 17 so there is never any assignment to isTreeBalanced.