Data structures play a key role in the programming world. They help us to organize our data in a way that can be used efficiently. The **stack** is one of the simplest data structures.

Let’s learn about the stack and its implementation in Python.

## What is a Stack?

A stack is similar to the pile of books, chairs, etc.., in real-life. And it follows the **Last-in/First-out (LIFO)** principle. It is the simplest data structure. Let’s see the scenario with a real-world example.

Let’s say we have a pile of books as follows.

When we want the third book from the top then, we have to remove the first two books from the top to take out the third book. Here, the topmost book goes last to the pile and comes first of the pile. The data structure **stack** follows the same principle **Last-in/First-out (LIFO)** in programming.

## Operations in Stack

There are mainly two operations in a stack

### 1. push(data)

Adds or pushes the data into the stack.

### 2. pop()

Removes or pops the topmost element from the stack.

See the below illustrations of **push** and **pop** operations.

We will write some helper functions that help us to get more info about the stack.

Let’s see them.

### peek()

Returns the topmost element from the stack.

### is_empty()

Returns whether the stack is empty or not.

Enough conceptual aspects of the **stack** data structure. Let’s jump into the implementation without further ado.

I assume you have python installed on your PC if not you can also try the online compiler.

## Stack Implementation

Implementing stack is the easiest one compared to other data structure implementations. We can implement a stack in multiple ways in Python.

Let’s see all of them one by one.

### #1. List

We are going to implement the stack using the list in a class. Let’s see the step by step implementation of the stack.

**Step1:** Write a class called Stack.

```
class Stack:
pass
```

**Step2:** We have to maintain the data in a list. Let’s add an empty list in the Stack class with name **elements**.

```
class Stack:
def __init__(self):
self.elements = []
```

**Step3:** To **push** the elements into the stack, we need a method. Let’s write a **push** method that takes data as an argument and **append** it to the **elements** list.

```
class Stack:
def __init__(self):
self.elements = []
def push(self, data):
self.elements.append(data)
return data
```

**Step4:** Similarly, let’s write the **pop** method that pops out the topmost element from the **stack**. We can use the **pop** method of the **list** data type.

```
class Stack:
## ...
def pop(self):
return self.elements.pop()
```

We have completed the stack implementation with the required operations. Now, let’s add the helper functions to get more info about the stack.

**Step5: **We can get the topmost element from the stack using the negative index. The code `element[-1]`

returns the last of the list. It is the topmost element of the stack in our case.

```
class Stack:
## ...
def peek(self):
return self.elements[-1]
```

**Step6:** If the length of the `elements`

list is **0**, then the stack is empty. Let’s write a method that returns whether the element is empty or not.

```
class Stack:
## ...
def is_empty(self):
return len(self.elements) == 0
```

We have completed implementing the stack using the **list** data type.

Oh! wait we just implemented it. But, didn’t see how to use it. How to use it then?

Come let’s see how to implement it. We have to create an object for the **Stack **class to use it. It’s not a big deal. Let’s do it first.

```
class Stack:
## ...
if __name__ == '__main__':
stack = Stack()
```

We have created the stack object and ready to use it. Let’s follow the below steps to test stack operations.

- Check whether the stack is empty or not using the
**is_empty**method. It should return**True**. - Push the numbers 1, 2, 3, 4, 5 into the stack using
**the push**method. - The
**is_empty**method should return**False**. Check it. - Print the topmost element using the
**peek**method. - Pop the element from the stack using the
**pop**method. - Check the peek element. It should return the element
**4**. - Now, pop all the elements from the stack.
- The
**is_empty**method should return**True**. Check it.

Our stack implementation is completed if it passes all the above steps. Try to write the code for the above steps.

Did you write the code? No, don’t worry check the code below.

```
class Stack:
def __init__(self):
self.elements = []
def push(self, data):
self.elements.append(data)
return data
def pop(self):
return self.elements.pop()
def peek(self):
return self.elements[-1]
def is_empty(self):
return len(self.elements) == 0
if __name__ == '__main__':
stack = Stack()
## checking is_empty method -> true
print(stack.is_empty())
## pushing the elements
stack.push(1)
stack.push(2)
stack.push(3)
stack.push(4)
stack.push(5)
## again checking is_empty method -> false
print(stack.is_empty())
## printing the topmost element of the stack -> 5
print(stack.peek())
## popping the topmost element -> 5
stack.pop()
## checking the topmost element using peek method -> 4
print(stack.peek())
## popping all the elements
stack.pop()
stack.pop()
stack.pop()
stack.pop()
## checking the is_empty method for the last time -> true
print(stack.is_empty())
```

Hurray! we have completed the stack implementation from scratch using the **list **data type. You will see the output as mentioned below if you run the above code.

```
True
False
5
4
True
```

We can directly use the **list **data type as a **stack**. The above implementation of stack helps you understand the stack implementation in other programming languages as well.

You can also check out these list related articles.

Let’s see the built-in deque from the **collections **built-in module which can act as a stack.

### #2. deque from collections

It is implemented as a double-ended queue. Since it supports the addition and removal of elements from both ends. Hence we can use it as a **stack**. We can make it follow the **LIFO** principle of the stack.

It is implemented using other data structures called the **doubly-linked** list. So the performance of the insertion and deletion of elements are consistent. Accessing elements from the middle linked list took **O(n) **time. We can use it as a **stack **as there is no need to access the middle elements from the stack.

Before implementing the stack, let’s see the methods that are used to implement the stack using the** queue**.

**append(data)**– used to push the data to the stack**pop()**– used to remove the topmost element from the stack

There are no alternative methods for **peek **and **is_empty**. We can print the whole stack in place of **peek **method. Next, we can use the** len **method to check whether the **stack **is empty or not.

Let’s implement the stack using **deque **from the **collections **module.

```
from collections import deque
## creating deque object
stack = deque()
## checking whether stack is empty or not -> true
print(len(stack) == 0)
## pushing the elements
stack.append(1)
stack.append(2)
stack.append(3)
stack.append(4)
stack.append(5)
## again checking whether stack is empty or not -> false
print(len(stack) == 0)
## printing the stack
print(stack)
## popping the topmost element -> 5
stack.pop()
## printing the stack
print(stack)
## popping all the elements
stack.pop()
stack.pop()
stack.pop()
stack.pop()
## checking the whether stack is empty or not for the last time -> true
print(len(stack) == 0)
```

That’s it. We have learned how to implement **stack **using the **deque** from the **collections **built-in module. You will get the output as mentioned below if you execute the above program.

```
True
False
deque([1, 2, 3, 4, 5])
deque([1, 2, 3, 4])
True
```

Till now, we have seen two ways to implement the stack. Are there any other ways to implement a stack? Yeah! Let’s see the final way to implement a stack in this tutorial.

### #3. LifoQueue

The name **LifoQueue** itself says that it follows the **LIFO **principle. Hence we can use it as a **stack **without any doubt. It is from the built-in module **queue**. The **LifoQueue **provides some handy methods that are useful in the stack implementation. Let’s see them

**put(data)**– adds or pushes the data to the queue**get()**– removes or pops the topmost element from the queue**empty()**– returns whether the stack is empty or not**qsize()**– returns the length of the queue

Let’s implement the stack using **LifoQueue **from the **queue** module.

```
from queue import LifoQueue
## creating LifoQueue object
stack = LifoQueue()
## checking whether stack is empty or not -> true
print(stack.empty())
## pushing the elements
stack.put(1)
stack.put(2)
stack.put(3)
stack.put(4)
stack.put(5)
## again checking whether stack is empty or not -> false
print(stack.empty())
## popping all the elements
print(stack.get())
print(stack.get())
print(stack.get())
## checking the stack size
print("Size", stack.qsize())
print(stack.get())
print(stack.get())
## checking the whether stack is empty or not for the last time -> true
print(stack.empty())
```

You will get the output mentioned below if you execute the above program without changing it.

```
True
False
5
4
3
Size 2
2
1
True
```

## Application of Stack

Now, you have sufficient knowledge about stacks to apply it in programming problems. Let’s see a problem and solve it using a stack.

Given an expression, write a program to check whether the parentheses, braces, curly-braces are balanced correctly or not.

Let’s see some examples.

**Input:** “[{}]([])”

**Output:** Balanced

**Input:** “{[}]([])”

**Output:** Not Balanced

We can use the stack to solve the above problem. Let’s see the steps to solve the problem.

- Create a stack to store the characters.
- If the length of the expression is odd, then the expression is
**Not Balanced** - Iterate through the given expression.
- If the current character is the opening bracket from
**( or { or [**, then push it to stack. - Else if the current character is a closing bracket
**) or } or ]**, then pop from the stack. - If the popped character is matching with the starting bracket then continue else brackets are not balanced.

- If the current character is the opening bracket from
- After the iteration, if the stack is empty then the equation is
**Balanced**else the equation is**Not****Balanced**.

We can make use of the set data type for brackets match checking.

```
## stack
class Stack:
def __init__(self):
self.elements = []
def push(self, data):
self.elements.append(data)
return data
def pop(self):
return self.elements.pop()
def peek(self):
return self.elements[-1]
def is_empty(self):
return len(self.elements) == 0
def balance_check(expression):
## checking the length of the expression
if len(expression) % 2 != 0:
## not balanced if the length is odd
return False
## for checking
opening_brackets = set('([{')
pairs = set([ ('(',')'), ('[',']'), ('{','}') ])
## stack initialization
stack = Stack()
## iterating through the given expression
for bracket in expression:
## checking whether the current bracket is opened or not
if bracket in opening_brackets:
## adding to the stack
stack.push(bracket)
else:
## popping out the last bracket from the stack
popped_bracket = stack.pop()
## checking whether popped and current bracket pair
if (popped_bracket, bracket) not in pairs:
return False
return stack.is_empty()
if __name__ == '__main__':
if balance_check('[{}]([])'):
print("Balanced")
else:
print("Not Balanced")
if balance_check('{[}]([])'):
print("Balanced")
else:
print("Not Balanced")
```

We can use the **stack **to solve many more problems. The above problem is one of them. Try to apply the stack concept wherever you think it best suits you.

### Conclusion

Yah! You have completed the tutorial. I hope you enjoyed the tutorial as much as I do while making it. That’s it for the tutorial.

Happy Coding 🙂 👨💻