Durga Pokharel

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# Making a Stack Data Type in Python

Originally Published at dataqoil.com.

## Introduction

Stack is one of the primitive data structure that we always have to study before diving into the Data Structure and Analysis. It is an example of ADT (Abstract Data Type) where operations are predefined. There are some other types of ADTs also like Queue, List etc.

## Operations

For any Data Type, most common operations includes inserting a data, removing a data and retrieving a data. For a simple stack there are 3 types of operations:

• Push: To insert a data on the top of a stack.
• Pop: To remove a data from the top of a stack.
• Top: To retrieve a top element data.

Stack operates in a LIFO (Last in First Out) way. Which means that at anytime, the pointer will be on the top of a stack and we are only allowed to operation with a data which is pointed by a pointer. If a stack is going to use an array then its size will be fixed but if we are going to use a list, then the size will variable.

### Push Operation

Lets assume that we are using an array for the stack with the size of 6 with data, `[4 5 6 8 5 2]`. Initially, a stack will be empty and the pointer will be pointing the top of stack's data which is bottom. Now in first step, to push a data, we will push 4 in the 0th position. Then pointer moves one step upwards. On the next step, next data is inserted on the 1st position. And at last, our stack will be full.

Lets write it on python.

``````class Stack:
def __init__(self, size):
self.size = size
self.storage = ["~"]*size
self.pointer = 0
print(f"New Stack: {self.storage}")
print(f"Pointer at {self.pointer}.\n")

def push(self, x):
self.storage[self.pointer] = x
print(f"New Stack: {self.storage}")
print(f"Pointer at {self.pointer+1}.\n")

self.pointer+=1

``````
``````data = [4, 5, 6, 8, 5, 2]
stack = Stack(size=6)

for x in data:
stack.push(x)
``````
``````New Stack: ['~', '~', '~', '~', '~', '~']
Pointer at 0.

New Stack: [4, '~', '~', '~', '~', '~']
Pointer at 1.

New Stack: [4, 5, '~', '~', '~', '~']
Pointer at 2.

New Stack: [4, 5, 6, '~', '~', '~']
Pointer at 3.

New Stack: [4, 5, 6, 8, '~', '~']
Pointer at 4.

New Stack: [4, 5, 6, 8, 5, '~']
Pointer at 5.

New Stack: [4, 5, 6, 8, 5, 2]
Pointer at 6.
``````

Now we have inserted the data into our stack but what if we inserted more data than the size of a stack? In that case, a overflow will happen and in above example, we have used a list as an storage and it will also show error if we tried to insert something.

``````stack.push(0)
``````
``````---------------------------------------------------------------------------

IndexError                                Traceback (most recent call last)

<ipython-input-23-c549502547a3> in <module>
----> 1 stack.push(0)

<ipython-input-21-dfbe390820f6> in push(self, x)
8
9     def push(self, x):
---> 10         self.storage[self.pointer] = x
11         print(f"New Stack: {self.storage}")
12         print(f"Pointer at {self.pointer+1}.\n")

IndexError: list assignment index out of range
``````

Which is expected but we must make error looking little bit different in our case.

``````class Stack:
def __init__(self, size):
self.size = size
self.storage = ["~"]*size
self.pointer = 0
print(f"New Stack: {self.storage}")
print(f"Pointer at {self.pointer}.\n")

def push(self, x):
try:
self.storage[self.pointer] = x
print(f"New Stack: {self.storage}")
print(f"Pointer at {self.pointer+1}.\n")

self.pointer+=1
except IndexError as e:
print(f"Overflow Occured at pointer: {self.pointer}")

``````
``````data = [4, 5, 6, 8, 5, 2, 0]
stack = Stack(size=6)

for x in data:
stack.push(x)
``````
``````New Stack: ['~', '~', '~', '~', '~', '~']
Pointer at 0.

New Stack: [4, '~', '~', '~', '~', '~']
Pointer at 1.

New Stack: [4, 5, '~', '~', '~', '~']
Pointer at 2.

New Stack: [4, 5, 6, '~', '~', '~']
Pointer at 3.

New Stack: [4, 5, 6, 8, '~', '~']
Pointer at 4.

New Stack: [4, 5, 6, 8, 5, '~']
Pointer at 5.

New Stack: [4, 5, 6, 8, 5, 2]
Pointer at 6.

Overflow Occured at pointer: 6
``````

### Pop Operation

Now we have our stack fully filled, lets remove values from it. The pop operation again works with data located on pointer's position.

``````class Stack:
def __init__(self, size):
self.size = size
self.storage = ["~"]*size
self.pointer = 0
print(f"New Stack: {self.storage}")
print(f"Pointer at {self.pointer}.\n")

def push(self, x):
try:
self.storage[self.pointer] = x
print(f"New Stack: {self.storage}")
print(f"Pointer at {self.pointer+1}.\n")

self.pointer+=1
except IndexError as e:
print(f"Overflow Occured at pointer: {self.pointer} \n")

def pop(self):
self.storage = self.storage[:-1]

print(f"New Stack: {self.storage}")
print(f"Pointer at {self.pointer-1}.\n")

self.pointer-=1
``````
``````data = [4, 5, 6, 8, 5, 2, 0]
stack = Stack(size=6)

for x in data:
stack.push(x)
stack.pop()
``````
``````New Stack: ['~', '~', '~', '~', '~', '~']
Pointer at 0.

New Stack: [4, '~', '~', '~', '~', '~']
Pointer at 1.

New Stack: [4, 5, '~', '~', '~', '~']
Pointer at 2.

New Stack: [4, 5, 6, '~', '~', '~']
Pointer at 3.

New Stack: [4, 5, 6, 8, '~', '~']
Pointer at 4.

New Stack: [4, 5, 6, 8, 5, '~']
Pointer at 5.

New Stack: [4, 5, 6, 8, 5, 2]
Pointer at 6.

Overflow Occured at pointer: 6

New Stack: [4, 5, 6, 8, 5]
Pointer at 5.
``````

Now that we have made a pop operation, what happens if our stack is empty and we tried to remove a data? That case is a stack underflow. Lets write that also.

``````for i in range(len(data)):
stack.pop()
``````
``````New Stack: [4, 5, 6, 8]
Pointer at 4.

New Stack: [4, 5, 6]
Pointer at 3.

New Stack: [4, 5]
Pointer at 2.

New Stack: [4]
Pointer at 1.

New Stack: []
Pointer at 0.

New Stack: []
Pointer at -1.

New Stack: []
Pointer at -2.
``````

In above code, no error is shown when pointer becomes -ve. Lets assume that if pointer is already 0 and we tried to remove, then it should be an error.

``````class Stack:
def __init__(self, size):
self.size = size
self.storage = ["~"]*size
self.pointer = 0
print(f"New Stack: {self.storage}")
print(f"Pointer at {self.pointer}.\n")

def push(self, x):
try:
self.storage[self.pointer] = x
print(f"New Stack: {self.storage}")
print(f"Pointer at {self.pointer+1}.\n")

self.pointer+=1
except IndexError as e:
print(f"Overflow Occured at pointer: {self.pointer} \n")

def pop(self):
if self.pointer<1:
print("Stack Underflow occured.")
else:
self.storage = self.storage[:-1]
print(f"New Stack: {self.storage}")
print(f"Pointer at {self.pointer-1}.\n")

self.pointer-=1

``````
``````data = [4, 5, 6, 8, 5, 2, 0]
stack = Stack(size=6)

for x in data:
stack.push(x)
stack.pop()

for i in range(len(data)):
stack.pop()
``````
``````New Stack: ['~', '~', '~', '~', '~', '~']
Pointer at 0.

New Stack: [4, '~', '~', '~', '~', '~']
Pointer at 1.

New Stack: [4, 5, '~', '~', '~', '~']
Pointer at 2.

New Stack: [4, 5, 6, '~', '~', '~']
Pointer at 3.

New Stack: [4, 5, 6, 8, '~', '~']
Pointer at 4.

New Stack: [4, 5, 6, 8, 5, '~']
Pointer at 5.

New Stack: [4, 5, 6, 8, 5, 2]
Pointer at 6.

Overflow Occured at pointer: 6

New Stack: [4, 5, 6, 8, 5]
Pointer at 5.

New Stack: [4, 5, 6, 8]
Pointer at 4.

New Stack: [4, 5, 6]
Pointer at 3.

New Stack: [4, 5]
Pointer at 2.

New Stack: [4]
Pointer at 1.

New Stack: []
Pointer at 0.

Stack Underflow occured.
Stack Underflow occured.
``````

Now tweaking it little bit more to make it more readable.

``````class Stack:
def __init__(self, size):
self.size = size
self.storage = ["~"]*size
self.pointer = 0
print(f"New Stack: {self.storage}")
print(f"Pointer at {self.pointer}.\n")

def push(self, x):
print("=" * 20+" Push Operation "+"=" * 20)
try:
self.storage[self.pointer] = x
print(f"New Stack: {self.storage}")
print(f"Pointer at {self.pointer+1}.")

self.pointer+=1
except IndexError as e:
print(f"Overflow Occured at pointer: {self.pointer}")
print("="*55 + "\n")

def pop(self):
print("=" * 20+" Pop Operation "+"=" * 20)
if self.pointer<1:
print("Stack Underflow occured.")
else:
self.storage = self.storage[:-1]
print(f"New Stack: {self.storage}")
print(f"Pointer at {self.pointer-1}.")

self.pointer-=1
print("="*55 + "\n")

``````
``````data = [4, 5, 6, 8, 5, 2, 0]
stack = Stack(size=6)

for x in data:
stack.push(x)
stack.pop()

for i in range(len(data)):
stack.pop()
``````
``````New Stack: ['~', '~', '~', '~', '~', '~']
Pointer at 0.

==================== Push Operation ====================
New Stack: [4, '~', '~', '~', '~', '~']
Pointer at 1.
=======================================================

==================== Push Operation ====================
New Stack: [4, 5, '~', '~', '~', '~']
Pointer at 2.
=======================================================

==================== Push Operation ====================
New Stack: [4, 5, 6, '~', '~', '~']
Pointer at 3.
=======================================================

==================== Push Operation ====================
New Stack: [4, 5, 6, 8, '~', '~']
Pointer at 4.
=======================================================

==================== Push Operation ====================
New Stack: [4, 5, 6, 8, 5, '~']
Pointer at 5.
=======================================================

==================== Push Operation ====================
New Stack: [4, 5, 6, 8, 5, 2]
Pointer at 6.
=======================================================

==================== Push Operation ====================
Overflow Occured at pointer: 6
=======================================================

==================== Pop Operation ====================
New Stack: [4, 5, 6, 8, 5]
Pointer at 5.
=======================================================

==================== Pop Operation ====================
New Stack: [4, 5, 6, 8]
Pointer at 4.
=======================================================

==================== Pop Operation ====================
New Stack: [4, 5, 6]
Pointer at 3.
=======================================================

==================== Pop Operation ====================
New Stack: [4, 5]
Pointer at 2.
=======================================================

==================== Pop Operation ====================
New Stack: [4]
Pointer at 1.
=======================================================

==================== Pop Operation ====================
New Stack: []
Pointer at 0.
=======================================================

==================== Pop Operation ====================
Stack Underflow occured.
=======================================================

==================== Pop Operation ====================
Stack Underflow occured.
=======================================================
``````

### Top Operation

This is a simple operation. It gives the value that is below the position currently pointed by a pointer.

``````class Stack:
def __init__(self, size):
self.size = size
self.storage = ["~"]*size
self.pointer = 0
print(f"New Stack: {self.storage}")
print(f"Pointer at {self.pointer}.\n")

def push(self, x):
print("=" * 20+" Push Operation "+"=" * 20)
try:
self.storage[self.pointer] = x
print(f"New Stack: {self.storage}")
print(f"Pointer at {self.pointer+1}.")

self.pointer+=1
except IndexError as e:
print(f"Overflow Occured at pointer: {self.pointer}")
print("="*55 + "\n")

def pop(self):
print("=" * 20+" Pop Operation "+"=" * 20)
if self.pointer<1:
print("Stack Underflow occured.")
else:
self.storage = self.storage[:-1]
print(f"New Stack: {self.storage}")
print(f"Pointer at {self.pointer-1}.")

self.pointer-=1
print("="*55 + "\n")

def top(self):
print("=" * 20+" Top Operation "+"=" * 20)
try:
print(f"Pointer at {self.pointer}.")
print(f"Return: {self.storage[self.pointer-1]}")
except:
print("Nothing on the top. Stack is empty.")
print("="*55 + "\n")

``````
``````data = [4, 5, 6, 8, 5, 2, 0]
stack = Stack(size=6)

for x in data:
stack.push(x)
stack.top()
stack.pop()

for i in range(len(data)):
stack.pop()
stack.top()
``````
``````New Stack: ['~', '~', '~', '~', '~', '~']
Pointer at 0.

==================== Push Operation ====================
New Stack: [4, '~', '~', '~', '~', '~']

Pointer at 1.

==================== Top Operation ====================
Pointer at 1.

Return: 4

==================== Push Operation ====================
New Stack: [4, 5, '~', '~', '~', '~']

Pointer at 2.

==================== Top Operation ====================
Pointer at 2.

Return: 5

==================== Push Operation ====================
New Stack: [4, 5, 6, '~', '~', '~']

Pointer at 3.

==================== Top Operation ====================
Pointer at 3.

Return: 6

==================== Push Operation ====================
New Stack: [4, 5, 6, 8, '~', '~']

Pointer at 4.

==================== Top Operation ====================
Pointer at 4.

Return: 8

==================== Push Operation ====================
New Stack: [4, 5, 6, 8, 5, '~']

Pointer at 5.

==================== Top Operation ====================
Pointer at 5.

Return: 5

==================== Push Operation ====================
New Stack: [4, 5, 6, 8, 5, 2]

Pointer at 6.

==================== Top Operation ====================
Pointer at 6.

Return: 2

==================== Push Operation ====================

Overflow Occured at pointer: 6

==================== Top Operation ====================
Pointer at 6.

Return: 2

==================== Pop Operation ====================
New Stack: [4, 5, 6, 8, 5]

Pointer at 5.

==================== Pop Operation ====================
New Stack: [4, 5, 6, 8]

Pointer at 4.

==================== Top Operation ====================
Pointer at 4.

Return: 8

==================== Pop Operation ====================
New Stack: [4, 5, 6]

Pointer at 3.

==================== Top Operation ====================
Pointer at 3.

Return: 6

==================== Pop Operation ====================
New Stack: [4, 5]

Pointer at 2.

==================== Top Operation ====================
Pointer at 2.

Return: 5

==================== Pop Operation ====================
New Stack: [4]

Pointer at 1.

==================== Top Operation ====================
Pointer at 1.

Return: 4

==================== Pop Operation ====================
New Stack: []

Pointer at 0.

==================== Top Operation ====================
Pointer at 0.

Nothing on the top. Stack is empty.

==================== Pop Operation ====================

Stack Underflow occured.

==================== Top Operation ====================
Pointer at 0.

Nothing on the top. Stack is empty.

==================== Pop Operation ====================

Stack Underflow occured.

==================== Top Operation ====================
Pointer at 0.

Nothing on the top. Stack is empty.

``````

## Conclusion

Thank you for reading this blog and reaching to the end. In the next blog, I will share similar approach for the Queue and it will also be a fun to try.

Vincent A. Cicirello • Edited

This is all technically correct but unnecessary in Python. For a stack in Python just use a list like this:

``````# Initialize empty stack as empty list

stack = [ ]

# For push just use append

stack.append("world")
stack.append("hello")

# For pop just use pop

x = stack.pop() if len(stack) > 0 else None

# For top just use an index of -1

top = stack[-1] if len(stack) > 0 else None
``````

For a queue, import the deque type (double ended queue) from the collections module and use that.

Durga Pokharel

Yeah you are right. Thank you for pointing out. My goal was to code the theory behind the Stack but not the fast and short codes. π

So it's unnecessary study and implement data structures in python cuz they are implemented already???? rofl

Vincent A. Cicirello

That is not at all what I said. I said nothing about data structures more generally. However, you wouldn't implement a stack in python the way described in this post. Instead, if you were implementing a stack in python to learn about stacks, you would have a much simpler stack class wrapping a python list with methods for the stack operations implemented the way I described. It still gets across main point of using right end as top of stack.

Using a python list as if it was instead a simple array, as described in this post, just encourages bad python style.

LuZzoO

But, the whole point is to understand what happens.