seek the lower ancestor in a binary tree

Question

I wanna find the lower ancestor in a binary tree, and what i do is first, list the fathers of each node, then compare the list and the last item in common is the lower ancestry.

I have this code:

def ancesters(self, node, list= []):
    if self.seekNode(node) != False:
        if node < self.id:
            list.append(self.id)
            return self.left.ancesters(node)
        elif node > self.id:
            list.append(self.id)
            return self.right.ancesters(node)
        elif self.id == node:
            list.append(self.id)
            return list
    else:
        return False

the function seekNode works, and this method too, but when i use the method twice, shows the list of ancestor of the last call, example:

i have this tree:

and when i call the method ancesters(6), the list will be (2,5,6), and when i call again to search the fathers of 3, shows (2,5,6,2,5,3).

so, when i set the parameter (list=[]) why the list does not initialize and save the list value?. I call the method with the same object, in this case will be the node (root) of the tree. the nodes are instance of the node (root) of the tree.

See https://stackoverflow.com/questions/1132941/least-astonishment-and-the-mutable-default-argument — Michael Butscher, Nov 21 '20 at 16:38
Don't use names from standard library (like "list") as variables or parameters. — Michael Butscher, Nov 21 '20 at 16:39

score 0 · Accepted Answer · answered Nov 21 '20 at 16:43

In short, do not use mutable object (like list) as a default value. This is Anti-Pattern, because in Python the default value is shared between all function calls. so it must be immutable. popular option is None and condition if not None: ...

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seek the lower ancestor in a binary tree

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