post fix expression evaluation

postfix01.py

'''
Created on 15/10/2014

@author: mvalle
'''
import unittest
from decimal import Decimal


OPS = {
    '+': Decimal.__add__,
    '-': Decimal.__sub__,
    '*': Decimal.__mul__,
    '/': Decimal.__div__,
}

def postfix_evaluate(expression):
    s = []
    operand = 0
    has_digit = False
    for c in expression.strip():
        if c == ' ':
            if has_digit:
                s.append(operand)
                operand = 0 
                has_digit = False           
        elif c.isdigit():
            operand = 10 * operand + int(c)
            has_digit = True            
        elif c in '*/+-':
            s.append(OPS[c](Decimal(s.pop()), Decimal(s.pop())))
        else:
            raise "Invalid char in expression"
            
    return s.pop()

class PostfixTest(unittest.TestCase):
    
    def test1(self):
        result = postfix_evaluate('2 3 * 5 +') 
        print "test1 result =", result       
        assert (result == 11)
        
        
    def test2(self):
        result = postfix_evaluate('2 3 * 6 + 15 20 10 + * /')
        print "test2 result =", result        
        assert (result == Decimal(37.5))


if __name__ == "__main__":
    # import sys;sys.argv = ['', 'Test.testName']
    unittest.main()