Stack implementation in Python (http://jackm.co.uk/posts/2014/04/12/evaluating-expressions-using-reverse-polish-notation/)

stack.py

class Stack:
  def __init__(self):
    # Empty stack represented with an empty list.
    self.stack = []
 
  def push(self, x):
    # Append the item onto our list.
    self.stack.append(x)
 
  def pop(self):
    # Pop the item off of our list.
    return self.stack.pop()
 
  def peek(self):
    # As long as it isn't empty, return the last item in our list.
    if len(self.stack) > 0:
      return self.stack[-1]   # Index -1 = last element.
    else:
      return None