RPN calculator kata

import unittest

'''
I want to develop a simple RPN-calculator (RPN = Reverse polish notation).
It shouldn't be interactive or anything, it should just take a string as
input and compute it.

For example>

1 2 +

.. should compute to 3.

It should only work with integers and basic arithmetics: +, -, * and /.
'''

def is_number(str):
    try: int(str)
    except: return False
    return True

def rpn(expr):
    parts = expr.split()
    stack = []
    def plus(l, r): return l + r
    def minus(l, r): return l - r
    def times(l, r): return l * r
    def through(l, r): return l / r
    words = {"+":plus, "-":minus, "*":times, "/":through}
    for part in parts:
        if is_number(part):
            stack.append(int(part))
        else:
            right_number = stack.pop()
            left_number = stack.pop()
            op = words[part]
            stack.append(op(left_number, right_number))

    if len(stack)<>1:
        raise Exception()
    return stack[0]

class TestCase(unittest.TestCase):
    def test_no_op_returns_number(self):
        self.assertEqual(1, rpn("1"))

    def test_no_op_returns_number2(self):
        self.assertEqual(2, rpn("2"))

    def test_putting_up_two_numbers_is_an_error(self):
        self.assertRaises(Exception, rpn, "2 3")

    def test_plus(self):
        self.assertEqual(5, rpn("2 3 +"))

    def test_plus2(self):
        self.assertEqual(6, rpn("2 4 +"))

    def test_two_pluses(self):
        self.assertEqual(9, rpn("2 4 + 3 +"))

    def test_minus(self):
        self.assertEqual(-2, rpn("2 4 -"))

    def test_minus2(self):
        self.assertEqual(0, rpn("2 2 -"))

    def test_plus_minus(self):
        self.assertEqual(3, rpn("2 2 + 1 -"))

    def test_tab_accepted(self):
        self.assertEqual(3, rpn("2\t2 + 1 -"))

    def test_double_space_accepted(self):
        self.assertEqual(3, rpn("2    2 + 1 -"))

    def test_times(self):
        self.assertEqual(10, rpn("2 5 *"))

    def test_through(self):
        self.assertEqual(6, rpn("12 2 /"))

    def test_through_rounds_down(self):
        self.assertEqual(6, rpn("13 2 /"))

    def test_through_rounds_down2(self):
        self.assertEqual(-7, rpn("-13 2 /"))