Python递归函数定义为内部函数

Question

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def pathSum(self, root, sum):
        """
        :type root: TreeNode
        :type sum: int
        :rtype: int
        """
        #ref http://bookshadow.com/weblog/2016/10/23/leetcode-path-sum-iii/
        def traverse(root, sum):
            """
            :rtype: int
            return the number of paths connecting to root and has sum equals to sum
            """
            if not root:
                return 0
            return (root.val==sum) + traverse(root.left, sum-root.val) + traverse(root.right, sum-root.val)


        if not root:
            return 0
        ans = 0

        ans += traverse(root, sum)
        #if not start from root
        ans += self.pathSum(root.left, sum) + self.pathSum(root.right, sum)

        return ans

在上面的python代码中，为什么在递归使用函数pathSum时，必须有self.pathSum，但是当使用内部函数＆＃34;遍历＆＃34;递归地，没有自我。＆＃34;递归调用遍历之前的前缀？