RPN Calculator in Python.

RPN Calculator to solve first Praxis challenge.
http://programmingpraxis.com/2009/02/19/rpn-calculator/

Fairly happy with it, I don't feel right using the in operator to evaluate the token against a string though, it is a bit of a kludge. The first version of this program used a ValueError exception to parse an operator. Although in my heart of hearts I knew this was a bad thing to do it still felt somehow....cleaner.

import sys, operator

def main():
  inplist = list(line.split() for line in sys.stdin.readlines())
  functions = {'+':operator.add, '-':operator.sub, '*':operator.mul, '/':operator.truediv}

  for expression in inplist:
      out = list()
      for token in expression:
          if token[0] in '01234567890':
              out.append(float(token))
          else:
              rhs = out.pop()
              lhs = out.pop()
              out.append(functions[token](lhs,rhs))
      print out[0]    

main()