rpn.py

#!/usr/bin/python

def rpn(stack, obj):
    if type(obj) == int:
        stack.append(obj)
    else:
        p = stack.pop()
        if obj == '+':
            stack.append(stack.pop() + p)
        elif obj == '-':
            stack.append(stack.pop() - p)
        elif obj == '*':
            stack.append(stack.pop() * p)
        elif obj == '/':
            stack.append(stack.pop() / p)
    return stack

while True:
    stack = []
    print("半角スペース区切りで問題を入力してください")
    formula = input("-> ").split()
    try:
        for f in formula:
            stack = rpn(stack, int(f) if f.isdigit() else f)
        print(stack[0])
    except:
        print('Error: invalid input')
        break