Python RPN calculator implementation

rpn.py

#!/usr/bin/env python3

import sys
import operator
from decimal import Decimal

def main():
    ops = {
        "+": operator.add,
        "-": operator.sub,
        "*": operator.mul,
        "/": operator.truediv
    }
    digit = set("0123456789.")
    stack = []

    for part in input("EXPR > ").strip().split(' '):
        if set(part).issubset(digit):
            stack.append(part)
        elif part in ops:
            if len(stack) < 2:
                print("Two operands needed")
                sys.exit(1)
            else:
                r = Decimal(stack.pop())
                l = Decimal(stack.pop())
                stack.append(ops[part](l, r))
        else:
            print("Invalid character: {}".format(part))
            sys.exit(1)

    print(stack.pop())

if __name__ == '__main__':
    main()

INPUT:

EXPR > 1.5 45 * 9 / 12 +

OUTPUT:

19.5