Queue in Python

Queue is a linear data structure that stores items in a First In First Out (FIFO) manner. The item that is added first will be removed first. Queues are widely used in real-life scenarios, like ticket booking, or CPU task scheduling, where first-come, first-served rule is followed.

Operations associated with queue are: 

• Enqueue: Adds an item to the queue. If queue is full, it is said to be an Overflow condition – Time Complexity : O(1)
• Dequeue: Removes an item from the queue. If the queue is empty, it is said to be an Underflow condition – Time Complexity : O(1)
• Front: Get front item from queue – Time Complexity : O(1)
• Rear: Get last item from queue – Time Complexity : O(1)

Implement a Queue

There are various ways to implement a queue in Python by following ways:

1. Implementation using list

Lists can be used as queues, but removing elements from front requires shifting all other elements, making it O(n).

Example: Simulate a queue with a Python list.

queue = []
queue.append('a')
queue.append('b')
queue.append('c')
print("Initial queue:", queue)

print("Elements dequeued from queue:")
print(queue.pop(0))
print(queue.pop(0))
print(queue.pop(0))

print("Queue after removing elements:", queue)

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

Explanation: We added elements using append() and removed from the front using pop(0). After removing all elements, queue is empty.

2. Implementation using collections.deque

deque (double-ended queue) is preferred over a list for queues because both append() and popleft() run in O(1) time.

Example: Queue using deque.

from collections import deque
q = deque()

q.append('a')
q.append('b')
q.append('c')
print("Initial queue:", q)

print("Elements dequeued from the queue:")
print(q.popleft())
print(q.popleft())
print(q.popleft())

print("Queue after removing elements:", q)

Initial queue: deque(['a', 'b', 'c'])
Elements dequeued from the queue:
a
b
c
Queue after removing elements: deque([])

Explanation: popleft() efficiently removes the first element without shifting, making deque ideal for queues.

3. Implementation using queue.Queue

Python’s queue module provides a thread-safe FIFO queue. You can specify a maxsize. Key Methods are:

• put(item) / put_nowait(item) – Add an element.
• get() / get_nowait() – Remove an element.
• empty() – Check if the queue is empty.
• full() – Check if the queue is full.
• qsize() – Get current size of the queue.

Example: Queue using queue.Queue.

from queue import Queue
q = Queue(maxsize=3)
print("Initial size:", q.qsize())

q.put('a')
q.put('b')
q.put('c')
print("Is full:", q.full())

print("Elements dequeued from the queue:")
print(q.get())
print(q.get())
print(q.get())
print("Is empty:", q.empty())

q.put(1)
print("Is empty:", q.empty())
print("Is full:", q.full())

Initial size: 0
Is full: True
Elements dequeued from the queue:
a
b
c
Is empty: True
Is empty: False
Is full: False

Explanation: queue.Queue class handles thread-safe operations. You can check fullness or emptiness before adding or removing elements.

To practice problems related to Queue, refer to this article Queue Data Structure