python 类对象的可变性何时影响赋值？

问：

我对 python 对类/对象的可变特性的理解是，如果您进行赋值，那么对原始变量的任何更改也会更改分配的变量/对象。我对下面的这段代码感到困惑。

# Recursive solution to Flatten Binary Tree to Linked List by LeetCode

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

class Solution:
    # @param root, a tree node
    # @return root, a tree node
    def flattenHelper(self, root):
        if root == None:
            return None
        else:
            left = root.left
            right = root.right
            root.left = None    # Truncate the left subtree
            current = root

            # Flatten the left subtree
            current.right = self.flattenHelper(left)
            while current.right != None:    current = current.right

            # Flatten the right subtree
            current.right = self.flattenHelper(right)
            return root

    # @param root, a tree node
    # @return nothing, do it in place
    def flatten(self, root):
        self.flattenHelper(root)
        return

问题：为什么变量不会自动设置为执行一次？leftNoneroot.left = None