Update array stack.

codes/python/chapter_stack_and_queue/array_stack.py

@@ -27,14 +27,17 @@ class ArrayStack:
 
     """ 出栈 """
     def pop(self):
+        assert not self.is_empty(), "栈为空"
         return self.__stack.pop()
 
     """ 访问栈顶元素 """
     def peek(self):
+        assert not self.is_empty(), "栈为空"
         return self.__stack[-1]
 
     """ 访问索引 index 处元素 """
     def get(self, index):
+        assert index < self.size(), "索引越界"
         return self.__stack[index]
     
     """ 返回列表用于打印 """