Infix expression Evaluation in Python

Infix expression Evaluation

import operator

ops = {'+':operator.add,'-':operator.sub,'*':operator.mul,'/':operator.div}

class Stack:
    def __init__(self):
        self.mylist = []

    def isempty(self):
        return True if len(self.mylist) == 0 \
            else False

    def top(self):
        if self.isempty():
            print "Stack is Empty"
            return 0
        else:
            return self.mylist[-1]

    def push(self,item):
        print "Pushing : %s" %item
        self.mylist.append(item)

    def pop(self):
        if not self.isempty():
            print "popping: %s" %self.top()
            return self.mylist.pop()
        else:
            print "Stack is Empty"
        return None


def do_infix_evaluation(items):
    print "****Evaluating the preceding items****",items
    items = map(lambda x: x if not isinstance(x,str) else int(x),items)
    print items
    firstitem = items.pop()
    prevresult = firstitem
    while firstitem:
        try:
            if hasattr(firstitem,'__call__'):
                prevresult = firstitem(prevresult,items.pop())

            firstitem = items.pop()
        except IndexError:
            return prevresult
        


    print "****** Evaluated Result is : %s" %prevresult
    return prevresult


def pop_until_openbrace(stack):
    tlist = []
    item = stack.pop()
    eres = 0
    while item != '(':
        if item:
            tlist.append(item)
            item = stack.pop()
        else:
            print "Insufficient items in stack"
            return

    return do_infix_evaluation(tlist) if tlist else eres


def infix(expression):
    print expression
    stack = Stack()
    for i in list(expression):
        if i == ')':
            stack.push(pop_until_openbrace(stack))
        elif ops.has_key(i):
            stack.push(ops[i])
        else:
            stack.push(i)

    print "Result is :",stack.mylist

if __name__ == '__main__':

    print "Enter expre:"
    expr = raw_input()
    infix(expr)

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