from __future__ import print_function
__author__ = 'Omkar Pathak'


class Stack(object):
 """ A stack is an abstract data type that serves as a collection of
 elements with two principal operations: push() and pop(). push() adds an
 element to the top of the stack, and pop() removes an element from the top
 of a stack. The order in which elements come off of a stack are
 Last In, First Out (LIFO).

 https://en.wikipedia.org/wiki/Stack_(abstract_data_type)
 """

 def __init__(self, limit=10):
 self.stack = []
 self.limit = limit

 def __bool__(self):
 return bool(self.stack)

 def __str__(self):
 return str(self.stack)

 def push(self, data):
 """ Push an element to the top of the stack."""
 if len(self.stack) >= self.limit:
 raise StackOverflowError
 self.stack.append(data)

 def pop(self):
 """ Pop an element off of the top of the stack."""
 if self.stack:
 return self.stack.pop()
 else:
 raise IndexError('pop from an empty stack')

 def peek(self):
 """ Peek at the top-most element of the stack."""
 if self.stack:
 return self.stack[-1]

 def is_empty(self):
 """ Check if a stack is empty."""
 return not bool(self.stack)

 def size(self):
 """ Return the size of the stack."""
 return len(self.stack)


class StackOverflowError(BaseException):
 pass


if __name__ == '__main__':
 stack = Stack()
 for i in range(10):
 stack.push(i)

 print('Stack demonstration:\n')
 print('Initial stack: ' + str(stack))
 print('pop(): ' + str(stack.pop()))
 print('After pop(), the stack is now: ' + str(stack))
 print('peek(): ' + str(stack.peek()))
 stack.push(100)
 print('After push(100), the stack is now: ' + str(stack))
 print('is_empty(): ' + str(stack.is_empty()))
 print('size(): ' + str(stack.size()))