CodeEval Challenge 7 (prefex solving )

prefix.py

#!/bin/python

from __future__ import division
import operator


class Slack:

    def __init__(self):
        self.items = []

    def isempty(self):
        return self.items == []

    def push(self, item):
        self.items.append(item)

    def pop(self):
        return self.items.pop()

    def size(self):
        return len(self.items)

    def peek(self):
        print self.items

d = {'+': operator.add,
     '-': operator.sub,
     '*': operator.mul,
     '/': operator.truediv,
     '%': operator.mod
     }

if __name__ == '__main__':
    op = Slack()
    val = Slack()

    s = '- + * 2 3 * 5 4 9'
    s = s.strip(' \t\r\n')
    l = [i for i in s.split(' ')]
    l.reverse()
    print l
    sum = 0
    for i in l:
        if not d.has_key(i):
            val.push(int(i))
            print "pushed i =>", i
        else:
            op.push(str(i))
            if val.size() >= 2:
	            try:
	                opt = str(op.pop())
	                print "opt is =>", opt
	                op1 = val.pop()
	                op2 = val.pop()
	                sum = d[opt](op1, op2)
	            except ZeroDivisionError:
	                sum = 0
	            val.push(sum)
        #print "sum is =>", sum
        
# print "current op is =>",op.peek()
# print "current val is =>",val.peek()

    print "sum =>", sum

The above code is not passing all the test cases. Need to fix something .. not sure what is it ?