import operator
 
OPERATORS = {
        '+': operator.add,
        '-': operator.sub,
        '*': operator.mul,
        '/': operator.div,
        }
 
class Stack(object):
    "Creates a stack to enter numbers or operators into"
 
    def __init__(self):
        self.storage = [0,0,0,0]
        self.sptn = -1        
 
    def push(self, item):
        self.sptn = self.sptn + 1
        self.storage[self.sptn] = item
 
    def pop(self):
        self.sptn = self.sptn -1
 
    def top(self):        
        return self.storage[self.sptn]
 
    def __rep__(self):
        return self.storage[:self.sptn + 1]
 
 
def get_input():
 
    equation = raw_input("Enter the equation in reverse polish notation: ")
    return equation
 
def evaluate_rpn(equation):
 
    s = Stack()
 
    try:
        for item in equation.split():
            try:
                result = float(item)
                s.push(result)
            except ValueError:
                x = s.pop()
                y = s.pop()
                result = OPERATORS[item](x, y)
                s.push(result)
        result = s.top()
 
    except IndexError:
        print "Please input at least two numbers before entering an operator."
 
def main():
    equation = get_input()
    answer = evaluate_rpn(raw_input("Enter the equation in reverse polish notation: "))
    print answer

import operator
 
OPERATORS = {
        '+': operator.add,
        '-': operator.sub,
        '*': operator.mul,
        '/': operator.div,
        }
 
class Stack(object):
    "Creates a stack to enter numbers or operators into"
 
    def __init__(self):
        self.storage = [0,0,0,0]
        self.sptn = -1
 
    def push(self, item):
        self.sptn = self.sptn + 1
        self.storage[self.sptn] = item
 
    def pop(self):
        self.sptn = self.sptn -1
 
    def top(self):
        return self.storage[self.sptn]
 
    def __rep__(self):
        return self.storage[:self.sptn + 1]
 
 
def get_input():
 
    equation = raw_input("Enter the equation in reverse polish notation: ")
    return equation
 
def evaluate_rpn(equation):
 
    s = Stack()
 
    try:
        for item in equation.split():
            try:
                result = float(item)
                s.push(result)
            except ValueError:
                x = s.pop()
                y = s.pop()
                print x, y   # test --> None None  problem is here!!!
                result = OPERATORS[item](x, y)
                s.push(result)
        result = s.top()
        print result  # test
        return result
 
    except IndexError:
        print "Please input at least two numbers before entering an operator."
 
def main():
    # use for testing
    equation = '2 3 +'
    #equation = get_input()
    answer = evaluate_rpn(equation)
    print answer
 
main()

import operator
import Queue
 
OPERATORS = {
        '+': operator.add,
        '-': operator.sub,
        '*': operator.mul,
        '/': operator.div,
        }
 
def get_input():
 
    equation = raw_input("Enter the equation in reverse polish notation: ")
    return equation
 
def evaluate_rpn(equation):
 
    # use a Python queue for a stack
    s = Queue.Queue(-1)
 
    try:
        for item in equation.split():
            try:
                result = float(item)
                s.put(result)
            except ValueError:
                x = s.get()
                y = s.get()
                #print x, y   # test
                result = OPERATORS[item](x, y)
                s.put(result)
        result = s.get()
        #print result  # test
        return result
 
    except IndexError:
        print "Please input at least two numbers before entering an operator."
 
def main():
    # use for testing
    equation = '2 3 +'
    # otherwise use ...
    #equation = get_input()
    answer = evaluate_rpn(equation)
    print answer  # '2 3 +' --> 5.0
 
main()