def test_trim_removes_double_link_between_parent_and_child_to_avoid_cycles(self):
parent = TreeNode(name = 'parent')
child = TreeNode(name = 'child')
parent.graft_child(child)
child.trim()
#assert child no longer in children of parent
child_ids_of_parent = [id(c) for c in parent.children]
child_ids_of_parent_spec = list()
self.assertItemsEqual(child_ids_of_parent, child_ids_of_parent_spec)
#assert parent is None for the child
self.assertIsNone(child.parent)
def test_trim_on_a_node_with_no_parent_does_nothing(self):
node = TreeNode()
node.trim()
self.assertIsNone(node.parent) #still None
