Stack and Queues in Python

Prerequisites : list and Deque in Python.

Unlike C++ STL and Java Collections, Python does have specific classes/interfaces for Stack and Queue. Following are different ways to implement in Python

1) Using list
Stack works on the principle of “Last-in, first-out”. Also, the inbuilt functions in Python make the code short and simple. To add an item to the top of the list, i.e., to push an item, we use append() function and to pop out an element we use pop() function. These functions work quiet efficiently and fast in end operations.

Let’s look at an example and try to understand the working of push() and pop() function:
Example:

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# Python code to demonstrate Implementing 
# stack using list
stack = ["Amar", "Akbar", "Anthony"]
stack.append("Ram")
stack.append("Iqbal")
print(stack)
  
# Removes the last item
print(stack.pop())
  
print(stack)
  
# Removes the last item
print(stack.pop())
  
print(stack)

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Output:



['Amar', 'Akbar', 'Anthony', 'Ram', 'Iqbal']
Iqbal
['Amar', 'Akbar', 'Anthony', 'Ram']
Ram
['Amar', 'Akbar', 'Anthony']

Queue works on the principle of “First-in, first-out”. Below is list implementation of queue. We use pop(0) to remove the first item from a list.

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# Python code to demonstrate Implementing 
# Queue using list
queue = ["Amar", "Akbar", "Anthony"]
queue.append("Ram")
queue.append("Iqbal")
print(queue)
  
# Removes the first item
print(queue.pop(0))
  
print(queue)
  
# Removes the first item
print(queue.pop(0))
  
print(queue)

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Output:

['Amar', 'Akbar', 'Anthony', 'Ram', 'Iqbal']
Amar
['Akbar', 'Anthony', 'Ram', 'Iqbal']
Akbar
['Anthony', 'Ram', 'Iqbal']

2) Using Deque
In case of stack, list implementation works fine and provides both append() and pop() in O(1) time. When we use deque implementation, we get same time complexity.

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# Python code to demonstrate Implementing 
# Stack using deque
from collections import deque
queue = deque(["Ram", "Tarun", "Asif", "John"])
print(queue)
queue.append("Akbar")
print(queue)
queue.append("Birbal")
print(queue)
print(queue.pop())                 
print(queue.pop())                 
print(queue)

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Output:

deque(['Ram', 'Tarun', 'Asif', 'John'])
deque(['Ram', 'Tarun', 'Asif', 'John', 'Akbar'])
deque(['Ram', 'Tarun', 'Asif', 'John', 'Akbar', 'Birbal'])
Birbal
Akbar
deque(['Ram', 'Tarun', 'Asif', 'John'])

But when it comes to queue, the above list implementation is not efficient. In queue when pop() is made from the beginning of the list which is slow. This occurs due to the properties of list, which is fast at the end operations but slow at the beginning operations, as all other elements have to be shifted one by one.
So, we prefer the use of queue over list, which was specially designed to have fast appends and pops from both the front and back end.

Let’s look at an example and try to understand queue using collections.deque:

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# Python code to demonstrate Implementing 
# Queue using deque
from collections import deque
queue = deque(["Ram", "Tarun", "Asif", "John"])
print(queue)
queue.append("Akbar")
print(queue)
queue.append("Birbal")
print(queue)
print(queue.popleft())                 
print(queue.popleft())                 
print(queue)

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Output:

deque(['Ram', 'Tarun', 'Asif', 'John'])
deque(['Ram', 'Tarun', 'Asif', 'John', 'Akbar'])
deque(['Ram', 'Tarun', 'Asif', 'John', 'Akbar', 'Birbal'])
Ram
Tarun
deque(['Asif', 'John', 'Akbar', 'Birbal'])

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