A reverse Polish notation calculator in 50 lines.

rpn.py

"""A reverse Polish notation calculator. Supports float and int literals."""
from __future__ import print_function
import operator
import sys

OPERATORS = {'+': operator.add, '-': operator.sub, '%': operator.mod, '/':
             operator.truediv, '//': operator.floordiv, '*': operator.mul,
             '**': operator.pow}


def recognize_numeric(token):
    """Recognize int and float literals. Return None if it's neither."""
    if token.isnumeric():
        return int(token)
    else:
        try:
            return float(token)
        except ValueError:
            return None


def parse_rpn(expr):
    """Execute reverse Polish notation arithmetic. ValueError on bad parse."""
    stack = []
    for token in expr.split():
        result = recognize_numeric(token)
        if result is None:
            try:
                b, a = stack.pop(), stack.pop()
                result = OPERATORS[token](a, b)
            except (KeyError, IndexError):
                raise ValueError
        stack.append(result)
    if len(stack) > 1:
        raise ValueError
    return stack.pop()


if __name__ == "__main__":
    while True:
        print("> ", end="")
        try:
            expr = input()
        except EOFError:
            print('Bye')
            sys.exit(0)
        try:
            print(parse_rpn(expr))
        except ValueError:
            print('Invalid parse')