      "            self.shift_stacks(self.right_stack, self.left_stack)\n",
      "        self.left_stack.push(data)\n",
      "\n",
      "    def dequeue(self):\n",
      "        if self.is_stack_empty(self.right_stack) and not self.is_stack_empty(self.left_stack):\n",
      "            self.shift_stacks(self.left_stack, self.right_stack)\n",
      "        return self.right_stack.pop()"
     ]
    },
    {
 
</s> remove     "        if self.is_stack_empty(self.left_stack) and not self.is_stack_empty(self.right_stack):\n",
 </s> add     "        if self.left_stack.is_empty() and not self.right_stack.is_empty():\n", </s> remove     "    def is_stack_empty(self, stack):\n",
    "        return stack.peek() is None\n",
    "\n",
 </s> add  </s> remove    "execution_count": 14,
 </s> add    "execution_count": null, </s> remove    "execution_count": 8,
 </s> add    "execution_count": null, </s> remove    "execution_count": 3,
 </s> add    "execution_count": null,
    "        if self.right_stack.is_empty() and not self.left_stack.is_empty():\n",
      "            self.shift_stacks(self.right_stack, self.left_stack)\n",
      "        self.left_stack.push(data)\n",
      "\n",
      "    def dequeue(self):\n",
    "        if self.right_stack.is_empty() and not self.left_stack.is_empty():\n",
      "            self.shift_stacks(self.left_stack, self.right_stack)\n",
      "        return self.right_stack.pop()"
     ]
    },
    {
keep keep keep keep replace keep keep keep keep keep
 Updated problem solution to use stack's is_empty() method rather than a similar method that used to be in QueueFromStacks. 
stacks-queues/queue-from-stacks.ipynb