Why global variable is giving an error: name 'po' is not defined. Did you mean:

Question

I am a python newbie, so question could be so simple sorry for that already.

class Solution:
    po = 0
    def buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:
        dic = {v : i for i, v in enumerate(inorder)}
        def helper(ios, ioe):
            global po
            if ios > ioe: return
            root = TreeNode(preorder[po])
            temp = dic[preorder[po]]
            po += 1
            root.left = helper(ios, temp - 1)
            root.right = helper(temp + 1, ioe)
            return root
        return helper(0, len(preorder) - 1)

My question in the above code is: I was trying to modify integer value po inside the function helper. I was using C++ before and in python unlike C++ I can't pass integers pass by reference so I though I could modify the global int variable instead. However, the code doesn't work. I get the following error: "name 'po' is not defined. Did you mean: 'pow'?"

The program works if I change the code like the following(po is inside buildTree now and instead of global nonlocal used):

class Solution:
    
    def buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:
        dic = {v : i for i, v in enumerate(inorder)}
        po = 0
        def helper(ios, ioe):
            nonlocal po
            if ios > ioe: return
            root = TreeNode(preorder[po])
            temp = dic[preorder[po]]
            po += 1
            root.left = helper(ios, temp - 1)
            root.right = helper(temp + 1, ioe)
            return root
        return helper(0, len(preorder) - 1)

My question is why global doesn't work but nonlocal works, can you explain by giving the underlying logic please?