In-order traversal of the tree

def in_order_traverse(self):
        """
        In-order traversal of the tree
        """
        result = []

        if not self.node:
            return result

        result.extend(self.node.left.in_order_traverse())
        result.append(self.node.key)
        result.extend(self.node.right.in_order_traverse())
        return result