Deque() in Python

Types of Restricted Deque Input

A deque is a data structure that allows you to insert and remove elements from both ends. But in a restricted deque input, there are some limitations on the type of operations that you can do.

There are two types of restricted deque input:

• Input-restricted deque: In this type of deque, you can only add elements to one end (the back end). But you can remove elements from both ends. It's like a queue where you can only add to the back of the line, but anyone can leave from the front or the back.
• Output-restricted deque: In this type of deque, you can only remove elements from one end (the front end). But you can add elements to both ends. It's like a stack where you can only take things off the top, but you can add things to the top or the bottom.

Both input and output-restricted deques have their own specific uses, depending on what you need to do with the data.

Operations on Python deque()

Also see, Python Operator Precedence

Deque In Python Using List

• Python

Python

# Showing the functions of a dequeue in python using a list
queue = []


# Enqueue at the right
queue.append(1)


# Enqueue at the right
queue.append(2)


# Enqueue at the left
queue.insert(0,3)


print("The Queue is:")
print(queue)


# Dequeue
print("The element popped from the queue is")


# Pop element from the left
print(queue.pop(0))
print("Updated Queue")
print(queue)


# Pop element from the right
print("The element popped from the queue is")
print(queue.pop(0))
print("Updated Queue")
print(queue)


'''
If the pop function is called on an empty queue, an error will be thrown
this is because there are no elements to access on the provided index
'''

You can also try this code with Online Python Compiler

Run Code

Output:

The Queue is:
[3, 1, 2]
The element popped from the queue is
3
Updated Queue
[1, 2]
The element popped from the queue is
1
Updated Queue
[2]

Python also has an inbuilt module, which helps implement deque in python. Thus, it must be imported from the python collections to utilize that module. The module is called deque. The implementation of deque in python from collections can be done as follows:

Deque In Python Using Collections

Let us look at some of the functions obtained for deque() in python from collections:

Now let us take a look at it's implementation:

• Python

Python

# Implementation of deque() in python from collections
 
from collections import deque


original_queue = []
n = int(input("Enter the Number of Elements in the Queue: "))
print("Enter the Numbers: ")


for i in range(n):
    temp = int(input())
    original_queue.append(temp)
dq = deque(original_queue)


# Insert element at the left of the deque
dq.appendleft(1)
dq.appendleft(2)


# Insert elements at the right of the deque
dq.append(3)
dq.append(4)


print("The data in the Deque is as follows:")
print(dq)
 
# Removing data from the left
print("Popped Element from the Left:")
print(dq.popleft())
print("Updated Deque")
print(dq)
 
# Removing data from the right
print("popped Element from the Right:")
print(dq.pop())
print("Updated Deque")
print(dq)

You can also try this code with Online Python Compiler

Run Code

Output:

Enter the Number of Elements in the Queue: 5
Enter the Numbers:
11
13
12
15
16
The data in the Deque is as follows:
deque([2, 1, 11, 13, 12, 15, 16, 3, 4])
Popped Element from the Left:
2
Updated Deque
deque([1, 11, 13, 12, 15, 16, 3, 4])
Popped Element from the Right:
4
Updated Deque
deque([1, 11, 13, 12, 15, 16, 3])

Apart from the major functions of enqueuing and dequeuing elements, deque() in python offers some extra functionality as well, such as:

Let us take a look at the implementation of the above-mentioned functions:

Additional Features For Deque In Python Using Collections

• Python

Python

# Implementation of deque() in python from collections


from collections import deque


dq = deque([1,2,3,3,3,4,5,5,6,7])


print("The Deque is:")
print(dq)
print("\n")


# Insert element at the left of the deque
dq.appendleft(1)
dq.appendleft(2)


# Insert elements at the right of the deque
dq.append(3)
dq.append(4)


print("The data in the Deque is as follows:")
print(dq)
print("\n")


# Removing data from the left
print("Popped Element from the Left:")
print(dq.popleft())
print("Updated Deque")
print(dq)
print("\n")


# Removing data from the right
print("popped Element from the Right:")
print(dq.pop())
print("Updated Deque")
print(dq)
print("\n")


print("The first occurrence of element at 3 between the index 1 and 6 is at:")
print(dq.index(3,1,6))
print("\n")


print("Inserting 10 at index 2:")
dq.insert(2,10)
print("Updated Deque:")
print(dq)
print("\n")


print("Removing first occurrence of 5")
dq.remove(5)
print("Updated Deque:")
print(dq)
print("\n")


print("The Number of occurrences of the element 3 are: ")
print(dq.count(3))
print("\n")


print("Adding the list [0,-1,-2] to the right of the deque")
dq.extend([0,-1,-2])
print("Updated Deque")
print(dq)
print("\n")


print("Adding the list [10,11,12] to the left of the deque")
dq.extendleft([10,11,12])
print("Updated Deque")
print(dq)
print("\n")


print("Reversing the Deque")
dq.reverse()
print("Updated Deque")
print(dq)
print("\n")


print("Rotating the Deque to the left by 3")
dq.rotate(3)
print("Updated Deque")
print(dq)
print("\n")

You can also try this code with Online Python Compiler

Run Code

Output:

The Deque is:
deque([1, 2, 3, 3, 3, 4, 5, 5, 6, 7])


The data in the Deque is as follows:
deque([2, 1, 1, 2, 3, 3, 3, 4, 5, 5, 6, 7, 3, 4])


Popped Element from the Left:
2
Updated Deque
deque([1, 1, 2, 3, 3, 3, 4, 5, 5, 6, 7, 3, 4])


popped Element from the Right:
4
Updated Deque
deque([1, 1, 2, 3, 3, 3, 4, 5, 5, 6, 7, 3])


The first occurrence of element at 3 between the index 1 and 6 is at:
3


Inserting 10 at index 2:
Updated Deque:
deque([1, 1, 10, 2, 3, 3, 3, 4, 5, 5, 6, 7, 3])


Removing first occurrence of 5
Updated Deque:
deque([1, 1, 10, 2, 3, 3, 3, 4, 5, 6, 7, 3])


The Number of occurrences of the element 3 are: 
4


Adding the list [0,-1,-2] to the right of the deque
Updated Deque
deque([1, 1, 10, 2, 3, 3, 3, 4, 5, 6, 7, 3, 0, -1, -2])


Adding the list [10,11,12] to the left of the deque
Updated Deque
deque([12, 11, 10, 1, 1, 10, 2, 3, 3, 3, 4, 5, 6, 7, 3, 0, -1, -2])


Reversing the Deque
Updated Deque
deque([-2, -1, 0, 3, 7, 6, 5, 4, 3, 3, 3, 2, 10, 1, 1, 10, 11, 12])


Rotating the Deque to the left by 3
Updated Deque
deque([10, 11, 12, -2, -1, 0, 3, 7, 6, 5, 4, 3, 3, 3, 2, 10, 1, 1])

Building Efficient Queues With deque() in Python

We will understand this concept with a short example:

from collections import deque


queue = deque()
queue.append('apple')
queue.popleft()

This creates an empty queue using deque(), adds an element 'apple' to the queue using append(), and removes the first element from the queue using popleft(). The append() and popleft() methods have O(1) time complexity, making deque an efficient choice for implementing queues.

You can also read about the Multilevel Inheritance in Python.

Frequently Asked Questions

How is deque implemented python?

Deque is implemented as a doubly-linked list in Python. Each element in the deque holds a reference to the preceding and following items, allowing for constant time operations to add or remove elements from both ends of the deque.

Is deque better than queue in Python?

Yes, deque is often better than queue in Python for many tasks because it allows fast appends and pops from both ends. This makes deque versatile and faster for managing sequences, unlike queue, which is typically limited to FIFO operations.

Why is deque faster than list in Python?

deque is faster than a list for insertions and deletions at both ends due to its doubly-linked list structure. Unlike lists, which require shifting elements when modified at the start, deque can quickly access and adjust either end.

What is the difference between stack and deque?

A stack is a data structure with LIFO (last-in, first-out) behavior, typically allowing operations only at one end. In contrast, a deque supports fast insertions and deletions at both ends, making it suitable for both stack and queue operations.

Conclusion

In this article, we discussed Deque() in Python. The deque in Python is a powerful and flexible data structure that offers efficient appends and pops from both ends, making it ideal for a variety of use cases like implementing stacks, queues, and double-ended queues. With its optimized performance for front and back operations, deque is a go-to choice for scenarios requiring quick data manipulation at both ends of a sequence. 

Recommended Reading:

• Stack in Python
• Mergeable Stack
• Fibonacci Series in Python
• Merge Sort Python
• Convert String to List Python
• Python List Operations