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    Question

    A Python Queue class uses a list. Its is_empty method is

    implemented incorrectly. class Queue: ย  ย  def __init__(self): ย  ย  ย  ย  self._items = [] ย  ย  def enqueue(self, item): ย  ย  ย  ย  self._items.append(item) ย  ย  def dequeue(self): ย  ย  ย  ย  if not self.is_empty(): ย  ย  ย  ย  ย  ย  return self._items.pop(0) ย  ย  ย  ย  else: ย  ย  ย  ย  ย  ย  raise IndexError("Queue is empty") ย  ย  def is_empty(self): ย  ย  ย  ย  return self._items is None # Potential bug here If q = Queue() is created, and then q.dequeue() is called without any prior enqueue operations, what will happen?
    A It will correctly raise an IndexError("Queue is empty"). Correct Answer Incorrect Answer
    B It will return None. Correct Answer Incorrect Answer
    C It will raise an AttributeError. Correct Answer Incorrect Answer
    D It will raise a TypeError. Correct Answer Incorrect Answer
    E It will raise an IndexError because _items is an empty list, not None. Correct Answer Incorrect Answer

    Solution

    โ€ข Dry Run: o q = Queue(): q._items is initialized as an empty list []. o q.dequeue() is called: ๏‚ง It first checks if not self.is_empty():. ๏‚ง self.is_empty() evaluates self._items is None. ๏‚ง Since self._items is [] (an empty list) and not None, the expression [] is None is False. ๏‚ง Therefore, self.is_empty() returns False. ๏‚ง The if condition becomes if not False:, which is if True:. ๏‚ง The code inside the if block executes: return self._items.pop(0). ๏‚ง self._items is []. Calling pop(0) on an empty list raises an IndexError. โ€ข Why Correct Answer (E): It will raise an IndexError because _items is an empty list, not None. o This accurately describes the flow of execution and the resulting error. The is_empty method incorrectly returns False for an empty queue, leading to an attempt to pop from an empty list.

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