An implementation of http://www.cs.csi.cuny.edu/~zelikovi/csc326/data/assignment5.htm#_ftnref1

postfix.py

from __future__ import division, print_function, unicode_literals

import operator
import re
import sys


WHITESPACE_RE = re.compile(r'\s+')
NUMBER_RE = re.compile(r'-?\d+(\.\d+)?')
OPERATORS = {
    '+': operator.add,
    '-': operator.sub,
    '*': operator.mul,
    '/': operator.truediv,
    '^': operator.pow,
}


def parse_input(s):
    return [n.strip() for n in WHITESPACE_RE.split(s)]


def process_operator(stack, op):
    oper = OPERATORS.get(op)
    if not oper:
        raise Exception('Invalid operation: %s' % op)
    right = stack.pop()
    left = stack.pop()
    result = oper(left, right)
    stack.append(result)


def do_math(formula):
    stack = []

    for n in parse_input(formula):
        if NUMBER_RE.match(n):
            stack.append(float(n))
        elif n in OPERATORS:
            process_operator(stack, n)
        elif n == '=':
            return stack[0]

    raise Exception("No '=' in %s" % formula)


def main():
    print('Enter an expression or # to quit.')

    while True:
        line = raw_input('> ')

        line = line.strip()
        if line == '#':
            break

        try:
            print('%g' % do_math(line))
        except Exception as exc:
            print('Error!', exc)

    print('Goodbye!')


if __name__ == '__main__':
    main()