class Stack(object):
def __init__(self):
self._stack = []
def is_empty(self):
return self._stack == []
def push(self, elem):
self._stack.append(elem)
def pop(self):
if self._stack == []:
raise StackUnderflow("栈中没有元素!")
return self._stack.pop()
def top(self):
if self._stack == []:
raise StackUnderflow("栈中没有元素")
return self._stack[-1]
class StackUnderflow(ValueError):
pass