Like stacks, a queue stores n elements in a single-dimensional structure. The elements are added and removed in FIFO format. One end of the queue is called the rear and the other is called the front. When elements are removed from the front, the operation is called dequeue. When elements are added at the rear, the operation is called enqueue.

In the following diagram, the top portion shows the enqueue operation. Steps 1.1, 1.2, and 1.3 add three elements to the queue and the resultant queue is shown in 1.4. Note that Yellow is the rear and Red is the front.

The bottom portion of the following diagram shows a dequeue operation. Steps 2.2, 2.3, and 2.4 remove elements from the queue one by one from the front of the queue:

The queue shown in the preceding diagram can be implemented by using the following code:

class Queue(object):
   def __init__(self):
      self.items = []
   def isEmpty(self):
      return self.items == []
   def enqueue(self, item):
       self.items.insert(0,item)
   def dequeue(self):
      return self.items.pop()
   def size(self):
      return len(self.items)

Let's enqueue and dequeue elements as shown in the preceding diagram with the help of the following screenshot:

Note that the preceding code creates a queue first and then enqueues four items into it.