calc.py

def parse(s):
    s = s.split(' ')[::-1]
    stack = []
    for i in s:
        if i in ["+", "-", "*", "/"]:
            a, b = stack.pop(), stack.pop()
            if i == "+":
                stack.append(a + b)
            if i == "-":
                stack.append(a - b)
            if i == "*":
                stack.append(a * b)
            if i == "/":
                stack.append(a / b)
        else:
            stack.append(int(i))
    return stack[0]
        

assert parse("+ 1 / 4 2") == 3
assert parse("* - 10 5 2") == 10