new_contents: #!/usr/bin/env python
  # -*- coding: utf -8 -*-
  
  
  from stack import Stack
  
  
  def test_init_instance():
      stack1 = Stack()
      assert stack1._parent.head_node is None
  
  
  def test_push_1():
      stack1 = Stack()
      stack1.push(1)
      assert len(stack1) == 1 and stack1._parent.head_node.value == 1
  
  
  def test_push_3():
      stack1 = Stack([1, 2, 3])
      assert len(stack1) == 3 and stack1._parent.head_node.value == 3
  
  
  def test_pop():
      stack1 = Stack([1, 2, 3])
      assert stack1.pop().value == 3 and len(stack1) == 2
subject: clean up