python stack

#!/usr/bin/env python
# add stack datastructure
#

__author__ = "lynn lin"

class UnderFlowError(Exception):
    pass
class stack(object):
    def __init__(self):
        self.sta = []
    def __len__(self):
        return len(self.sta)
    def StackEmpty(self):
        return self.__len__()
    def push(self,value):
        self.sta.append(value)
    def pop(self):
        try:
            return self.sta.pop()
        except:
            raise UnderFlowError,"stack is empty ,no value to pop!"