foo.py

import operator
import itertools


def pairwise(iterable):
    orig, copy = itertools.tee(iterable)
    next(copy)
    return zip(orig, copy)


class Node:
    def __init__(self, value, left=None, right=None):
        assert value is not None
        self.value = value
        self.left = left
        self.right = right

    def __lt__(self, other):
        return self.value < other.value

    def traverse_inorder(self):
        if self.left:
            yield from self.left.traverse_inorder()
        yield self.value
        if self.right:
            yield from self.right.traverse_inorder()


def isBst(tree):
    values = tree.traverse_inorder()
    return all(itertools.starmap(operator.lt, pairwise(values)))