[Python] Why does assignment to outer variables not take effect in recursive algorithm?

I am trying to solve a interview question: how to convert a BST to a double linked list. Below is my code. I am really confused about how python recursion works. Why after the convert2DoubleLst_recursive , the prev and head remain to be None, I have assigned a new value to these two variables, why after this method call, I cannot save the change. I remembered that python pass arguments by reference, but why here I cannot save the changes. Many thanks in advance.

def convert2_recursive(node):
prev,head=None,None
convert2DoubleLst_recursive(node,prev,head)
end=head.lChild
printList(head,end)

def convert2DoubleLst_recursive(node,prev,head):
if node:
convert2DoubleLst_recursive(node.lChild,prev,head)
node.lChild=prev
if prev:
prev.rChild=node
else:
head=node
nextNode=node.rChild
head.lChild=node
node.rChild=head
prev=node
convert2DoubleLst_recursive(nextNode,prev,head)

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