Question
A Python Queue class uses a list. Its is_empty method is
implemented incorrectly. class Queue: Â Â def __init__(self): Â Â Â Â self._items = [] Â Â def enqueue(self, item): Â Â Â Â self._items.append(item) Â Â def dequeue(self): Â Â Â Â if not self.is_empty(): Â Â Â Â Â Â return self._items.pop(0) Â Â Â Â else: Â Â Â Â Â Â raise IndexError("Queue is empty") Â Â def is_empty(self): Â Â Â Â 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?Solution
• Dry Run: o q = Queue(): q._items is initialized as an empty list []. o q.dequeue() is called:  It first checks if not self.is_empty():.  self.is_empty() evaluates self._items is None.  Since self._items is [] (an empty list) and not None, the expression [] is None is False.  Therefore, self.is_empty() returns False.  The if condition becomes if not False:, which is if True:.  The code inside the if block executes: return self._items.pop(0).  self._items is []. Calling pop(0) on an empty list raises an IndexError. • 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.
