Queue in Python

Like a stack, the queue is a linear data structure that stores items in a First In First Out (FIFO) manner. With a queue, the least recently added item is removed first. A good example of a queue is any queue of consumers for a resource where the consumer that came first is served first.

Queue in Python

Operations associated with queue are:

Enqueue: Adds an item to the queue. If the queue is full, then it is said to be an Overflow condition – Time Complexity : O(1)
Dequeue: Removes an item from the queue. The items are popped in the same order in which they are pushed. If the queue is empty, then it is said to be an Underflow condition – Time Complexity : O(1)
Front: Get the front item from queue – Time Complexity : O(1)
Rear: Get the last item from queue – Time Complexity : O(1)

Implement a Queue in Python

There are various ways to implement a queue in Python. This article covers the implementation of queue using data structures and modules from Python library. Python Queue can be implemented by the following ways:

Implementation using list

List is a Python's built-in data structure that can be used as a queue. Instead of enqueue() and dequeue(), append() and pop() function is used. However, lists are quite slow for this purpose because inserting or deleting an element at the beginning requires shifting all of the other elements by one, requiring O(n) time.

The code simulates a queue using a Python list. It adds elements 'a', 'b', and 'c' to the queue and then dequeues them, resulting in an empty queue at the end. The output shows the initial queue, elements dequeued ('a', 'b', 'c'), and the queue's empty state.


queue = []
queue.append('a')
queue.append('b')
queue.append('c')
print("Initial queue")
print(queue)
print("\\nElements dequeued from queue")
print(queue.pop(0))
print(queue.pop(0))
print(queue.pop(0))
print("\\nQueue after removing elements")
print(queue)

Output:

Initial queue
['a', 'b', 'c']
Elements dequeued from queue
a
b
c
Queue after removing elements
[]