old_contents: """
  This is a simple implementation of the Stack data structure
  Use it as you please ^_^
  """
  
  class Stack():
      """
      Simple LIFO Stack data structure
      """
  
      def __init__(self):
          """Constructor declaring the private variable"""
          self._items = []
      
      def is_empty(self):
          """Check the emptiness of the stack"""
          return len(self._items) == 0
  
      def size(self):
          """Get the number of items in the stack"""
          return len(self._items)
  
      def push(self, item):
          """Push an item to the stack"""
          self._items.append(item)
  
      def pop(self):
          """Pop an item from the end of the stack"""
          if self.is_empty():
              raise RuntimeError("Attempt to pop an empty Stack!")
          return self._items.pop()
  
      def show(self):
          """Show the content of the stack"""
          res = "Stack (["
          if self.is_empty():
              print(res + "])")
              return
          for i in range(len(self._items)-1):
              res += str(self._items[i]) + ', '
          res += str(self._items[len(self._items)-1]) + "])"
          print(res)
          return
  
  
  def main():
      stack = Stack()
  
      stack.show()          # Stack ([])
      stack.push(42)        # At this point, stack looks like this Stack ([42])
      stack.push(23)        # At this point, stack looks like this Stack ([42, 23])
      stack.push(5)         # At this point, stack looks like this Stack ([42, 23, 5])
      print(stack.size())   # 3
      stack.show()          # Stack ([42, 23, 5])
      print(stack.pop())    # 5
      stack.show()          # Stack ([42, 23])
      print(stack.pop())    # 23
      print(stack.size())   # 1
      stack.show()          # Stack ([42])
      print(stack.pop())    # 42
      stack.show()          # Stack ([])
      print(stack.pop())    # Underflow condition, should raise the RuntimeError exception and print "Attempt to pop an empty Stack!"
  
  
  if __name__ == "__main__":
      main()
  
new_contents: """
  This is a simple implementation of the Stack data structure
  Use it as you please ^_^
  """
  
  class Stack():
      """
      Simple LIFO Stack data structure
      """
  
      def __init__(self):
          """Constructor declaring the private variable"""
          self._items = []
      
      def is_empty(self):
          """Check the emptiness of the stack"""
          return len(self._items) == 0
  
      def size(self):
          """Get the number of items in the stack"""
          return len(self._items)
  
      def push(self, item):
          """Push an item to the stack"""
          self._items.append(item)
  
      def pop(self):
          """Pop an item from the end of the stack"""
          if self.is_empty():
              raise RuntimeError("Attempt to pop an empty Stack!")
          return self._items.pop()
  
      def show(self):
          """Show the content of the stack"""
          res = "Stack (["
          if self.is_empty():
              print(res + "])")
              return
          for i in range(len(self._items)-1):
              res += str(self._items[i]) + ', '
          res += str(self._items[len(self._items)-1]) + "])"
          print(res)
          return
  
  
  def main():
      stack = Stack()
  
      stack.show()          # Stack ([])
      stack.push(42)        # At this point, stack looks like this: Stack ([42])
      stack.push(23)        # At this point, stack looks like this: Stack ([42, 23])
      stack.push(5)         # At this point, stack looks like this: Stack ([42, 23, 5])
      print(stack.size())   # 3
      stack.show()          # Stack ([42, 23, 5])
      print(stack.pop())    # 5
      stack.show()          # Stack ([42, 23])
      print(stack.pop())    # 23
      print(stack.size())   # 1
      stack.show()          # Stack ([42])
      print(stack.pop())    # 42
      stack.show()          # Stack ([])
      print(stack.pop())    # Underflow condition, should raise the RuntimeError exception and print "Attempt to pop an empty Stack!"
  
  
  if __name__ == "__main__":
      main()
  
subject: Update stack.py