text: """
  The base class of key value storage.
  """
  
  
  class BaseKeyValueStorage(object):
      """
      The storage of key-values.
      """
      def add(self, category, key, value=None):
          """
          Add a new attribute. If the key already exists, raise an exception.
  
          Args:
              category: (string, int) the category of data.
              key: (string) attribute's key.
              value: (string) attribute's value.
          """
          pass
  
      def save(self, category, key, value=None):
          """
          Set an attribute.
  
          Args:
              category: (string, int) the category of data.
              key: (string) attribute's key.
              value: (string) attribute's value.
          """
          pass
  
      def has(self, category, key):
          """
          Check if the attribute exists.
  
          Args:
              category: (string, int) the category of data.
              key: (string) attribute's key.
          """
          pass
  
      def all(self):
          """
          Get all data.
          :return:
          """
          pass
  
      def load(self, category, key, *default):
          """
          Get the value of an attribute.
  
          Args:
              category: (string, int) the category of data.
              key: (string) attribute's key.
              default: (any or none) default value.
  
          Raises:
              KeyError: If `raise_exception` is set and no matching Attribute
                  was found matching `key` and no default value set.
          """
          pass
  
      def load_category(self, category):
          """
          Get all a category's data.
  
          Args:
              category: (string) category's name.
          """
          pass
  
      def delete(self, category, key):
          """
          delete an attribute of an object.
  
          Args:
              category: (string, int) the category of data.
              key: (string) attribute's key.
          """
          pass
  
      def delete_category(self, category):
          """
          Remove all values of a category.
  
          Args:
              category: (string) the category of data.
          """
          pass
  
      def atomic(self):
          """
          Guarantee the atomic execution of a given block.
          """
          pass
  