Python 中对局部和全局变量 scope 的说明

Question

I'm a bit confused regarding local and global variables referenced outside a function scope. For one problem, I had this code and it worked:

def leafSimilar(self, root1: TreeNode, root2: TreeNode) -> bool:
    l1 = []
    l2 = []
        
    def traverseLeaf(root: TreeNode, sequence: list): 
        if not root: return 
        elif not root.left and not root.right: sequence.append(root.val)
        else: 
            traverseLeaf(root.left, sequence)
            traverseLeaf(root.right, sequence)
                
    traverseLeaf(root1, l1)
    traverseLeaf(root2, l2)
        
    return l1 == l2

You can see I reference the list object outside the function scope. However, if I try this:

# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def convertBST(self, root: TreeNode) -> TreeNode:
        total = 0
               
        def rightOrderTraversal(node: TreeNode):
            if not node: return
            elif not node.left and not node.right: 
                total += node.val
                node.val = total
            else: 
                rightOrderTraversal(node.right)
                total += node.val
                node.val = total
                rightOrderTraversal(node.left)                
            
        rightOrderTraversal(root)
        return root

It does not work, and the error says I reference total as a local variable when calling rightOrderTraversal . What kind of variables does Python create local references for? From these examples, it seems like Python references lists outside the function scope but not ints.

I also tried to make total global as a way to solve the issue, but that didn't work.

Answer 1

If you want to use a variable that is visible to any methods/sub functions within an object instance, you use the "self" to reference to the object instance, eg:

class Solution:
   
    def my_method(self, total):
        self.total = total
               
        def my_function(total):
            self.total += total

        my_function(total)

s = Solution()
s.my_method(1)
print(s.total)