How to check if a red-black tree is balanced

class Node:
  def __init__(self, value, color=RED):
    self.value = value
    self.left = None
    self.right = None
    self.color = color
class RedBlackTree:
  def __init__(self):
    self.root = None
  
  def _numBlack(self, root):
    if not root:
      return 0
    black_left = self._numBlack(root.left)
    black_right = self._numBlack(root.right)
    if black_left != black_right or black_left == -1:
      return -1
    if root.color == BLACK:
      return 1 + black_left
    else:
      return 0 + black_left
  def isBalanced(self):
    if not self.root or self.root.color != BLACK:
      return False
    black_left = self._numBlack(self.root.left)
    black_right = self._numBlack(self.root.right)
    if black_left != black_right or black_left == -1:
      return False
    return True
tree = RedBlackTree()
tree.root = Node(3, BLACK)
tree.root.right = Node(4, RED)
tree.root.left = Node(2, RED)
tree.root.left.right = Node(2.5, BLACK)
print("The tree is", "balanced" if tree.isBalanced() else "not balanced")