Reverse Polish Notation (python)

rpn.py

OPERATORS = {
    '*': lambda a, b: a * b,
    '/': lambda a, b: a / b,
    '+': lambda a, b: a + b,
    '-': lambda a, b: a - b
}


def polish_eval(exp):
    exp_list = exp.split(' ')
    stack = []
    for item in exp_list:
        if item in OPERATORS:
            b = stack.pop()
            a = stack.pop()
            result = OPERATORS[item](a, b)
            stack.append(result)
        else:
            stack.append(int(item))
    return stack.pop()

test_rpn.py

from unittest import TestCase
from rpn import polish_eval


class TestReversePolishNotation(TestCase):
    def test_simple_addition(self):
        self.assertEqual(47, polish_eval('12 35 +'))
        self.assertEqual(4, polish_eval('2 2 +'))
        self.assertEqual(1, polish_eval('-1 2 +'))

    def test_simple_subtraction(self):
        self.assertEqual(1, polish_eval('2 1 -'))
        self.assertEqual(5, polish_eval('2 -3 -'))
        self.assertEqual(1, polish_eval('-1 -2 -'))

    def test_simple_multiplication(self):
        self.assertEqual(4, polish_eval('2 2 *'))
        self.assertEqual(9, polish_eval('-3 -3 *'))
        self.assertEqual(-8, polish_eval('-2 4 *'))

    def test_simple_division(self):
        self.assertEqual(4, polish_eval('8 2 /'))
        self.assertEqual(-2, polish_eval('-4 2 /'))
        self.assertEqual(3, polish_eval('-6 -2 /'))

    def test_multi_operators(self):
        self.assertEqual(4, polish_eval('3 4 - 5 +'))
        self.assertEqual(14, polish_eval('5 1 2 + 4 * + 3 -'))