#!/usr/bin/env python
 
import logging
import re
import unittest
 
logging.basicConfig(level=logging.INFO)
 
operators_table = {'+': int.__add__, 
             '-': int.__sub__,
             '*': int.__mul__,
             '/': int.__div__,
             '^': int.__pow__}
 
class ExpressionError(Exception):
    def __init__(self, message):
        self._message = "Expression error: %s" % message
    def _get_message(self): 
        return self._message
    message = property(_get_message)
 
class TestEvaluation(unittest.TestCase):
    def test_correct(self):
        self.assertEqual(666, evaluate_postfix_expr("666"))
        self.assertEqual(2+3-6, evaluate_postfix_expr("2 3 + 6 -"))
        self.assertEqual(2*3+4, evaluate_postfix_expr("2 3 * 4 +"))
        self.assertEqual(2*(3+4), evaluate_postfix_expr("2 3 4 + *"))
        self.assertEqual(3**4, evaluate_postfix_expr("3   3  *     3  *      3 *"))
        self.assertEqual((7/2)**4, evaluate_postfix_expr("7 2 / 4 ^"))
        self.assertEqual((2**3)**4, evaluate_postfix_expr("2 3 ^ 4 ^"))
        self.assertEqual(5+((1+2)*4)-3, evaluate_postfix_expr("5 1 2 + 4 * 3 - +"))
 
    def test_malformed(self):
        self.assertRaises(ExpressionError, evaluate_postfix_expr, "+")
        self.assertRaises(ExpressionError, evaluate_postfix_expr, "2 +")
        self.assertRaises(ExpressionError, evaluate_postfix_expr, "+ 2 2")
        self.assertRaises(ExpressionError, evaluate_postfix_expr, "2 2")
        self.assertRaises(ExpressionError, evaluate_postfix_expr, "2 2 + -")
        self.assertRaises(ExpressionError, evaluate_postfix_expr, "a 2 -")
 
def evaluate_postfix_expr(expr):
    atoms = re.split(r"\s+", expr)
    stack = [] 
    for atom in atoms:
        if atom in ["+", "-", "*", "/", "^"]:
            try:
                op2 = stack.pop()
                op1 = stack.pop()
            except IndexError:
                raise ExpressionError("Too few operands (unbalanced)")
            logging.debug("Calculating %d %s %d" % (op1, atom, op2))
            atom = operators_table[atom](op1, op2)
        else:
            try:
                atom = int(atom)
            except ValueError:
                raise ExpressionError("Unable to parse '%s' as integer" % atom)
 
        try:
            stack.append(atom)
        except MemoryError:
            raise ExpressionError("Too long expression")
 
        logging.debug("Pushed element %d. Stack status: %s" % (atom, stack))
 
    if len(stack) == 1:
        return stack.pop()
    else:
        raise ExpressionError("Too many operands (unbalanced)")
 
if __name__ == "__main__":
    unittest.main()