Inorder successor in a binary tree

successor.py

def successor(node):
    if node.right:
        c = node.right
        while c.left:
            c = c.left
        return c
    
    n, p = node, node.parent
    # note: check if p is not None
    while p and n != p.left:
        n, p = p, p.parent
    return p