Dumb pure RPN calculator in Python

rpn.py

import operator

def do_op(op, lhs, rhs):
  return \
  {'+': operator.add,
   '-': operator.sub,
   '*': operator.mul}[op](lhs, rhs)

def rpn(lst, stack):
  if lst == []:
    return stack

  if lst[0] in '-+*':
    return rpn(lst[1:], [do_op(lst[0], stack[1], stack[0])] + stack[2:])

  return rpn(lst[1:], [int(lst[0])] + stack)

print rpn("90 34 12 33 55 66 + * - + -".split(), [])