4.12. Implementing a Queue in PythonΒΆ

It is again appropriate to create a new class for the implementation of the abstract data type queue. As before, we will use the power and simplicity of the list collection to build the internal representation of the queue.

We need to decide which end of the list to use as the rear and which to use as the front. The implementation shown in Listing 1 assumes that the rear is at position 0 in the list. This allows us to use the insert function on lists to add new elements to the rear of the queue. The pop operation can be used to remove the front element (the last element of the list). Recall that this also means that enqueue will be O(n) and dequeue will be O(1).

Listing 1

class Queue:
    def __init__(self):
        self.items = []

    def isEmpty(self):
        return self.items == []

    def enqueue(self, item):

    def dequeue(self):
        return self.items.pop()

    def size(self):
        return len(self.items)

CodeLens 1 shows the Queue class in action as we perform the sequence of operations from Table 1.

CodeLens: Example Queue Operations (ququeuetest)

Further manipulation of this queue would give the following results:

>>> q.size()
>>> q.isEmpty()
>>> q.enqueue(8.4)
>>> q.dequeue()
>>> q.dequeue()
>>> q.size()

Self Check

    Q-1: Suppose you have the following series of queue operations.

    q = Queue()

    What items are left on the queue?

  • 'hello', 'dog'
  • Remember the first thing added to the queue is the first thing removed. FIFO
  • 'dog', 3
  • Yes, first in first out means that hello is gone
  • 'hello', 3
  • Queues, and Stacks are both data structures where you can only access the first and the last thing.
  • 'hello', 'dog', 3
  • Ooops, maybe you missed the dequeue call at the end?
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