Max Number of Bends in Tree

Given a tree find maximum number of bends that a path can form

class TreeNode(object):
    def __init__(self,data):
        self.value = data
        self.left = None
        self.right = None

def find_path_with_max_bends(root):
    if not root:
        return 0
    left = _path_with_max_bends(root.left,'l')
    right = _path_with_max_bends(root.right,'r')
    return max(left,right)-1

def _path_with_max_bends(root,bend):
    if not root:
        return 0
    left_bends = _path_with_max_bends(root.left,'l')
    right_bends = _path_with_max_bends(root.right,'r')
    if bend == 'r':
        return max(1+left_bends,right_bends)
    else:
        return max(left_bends,1+right_bends)