Inorder successor, recursive

bst_successor.py

class Node:
    def __init__(self, value, left=None, right=None):
        self.value = value
        self.left = left
        self.right = right


def successor(node, x):
    """ Return smallest node greater than x, if it exists,
        in the subtree rooted at given node """
    
    if node is None:
        return None

    if node.value > x:
        s = successor(node.left, x)
        if s is None:
            s = node
        return s
    else:
        return successor(node.right, x)