reverse_polish.py

def calc(expr):
    operators = ['+', '-', '*', '/']
    stack = list()
    for item in expr.split(' '):
        if item in operators:
            right, left = [str(i) for i in (stack.pop(), stack.pop())]
            stack.append(eval(left + item + right))
        else:
            stack.append(eval(item + '+ 0'))

    return stack[-1]