gistfile1.py

import ast
import operator as op
import re
import sys

operators = {'+' : op.add,
             '-' : op.sub,
             '*' : op.mul,
             '/' : op.div}
             
def is_operator(x):
    return x in operators.keys()

def is_number(x):
    return bool(re.match(r'(\d*\.)?\d+', x))

def eval_rpn(tokens, stack=[]):
    if not tokens:
        return stack
    token = tokens[0]
    if is_operator(token):
        rh, lh = stack.pop(), stack.pop()
        stack.append(operators[token](lh, rh))
        return eval_rpn(tokens[1:], stack)
    elif is_number(token):
        stack.append(ast.literal_eval(token))
        return eval_rpn(tokens[1:], stack)
    else:
        raise Exception("Invalid token: %s"%token)

if __name__ == "__main__":
    print eval_rpn(sys.argv[1].split(" "))