code: def add_user(self, attrs):
          
          username = attrs[self.key]
          if username in self.users:
              raise UserAlreadyExists(username, self.backend_name)
          self.users[username] = attrs
          self.users[username]['groups'] = set([])
docs: Add a user to the backend
  
          :param attrs: attributes of the user
          :type attrs: dict ({<attr>: <value>})
  
          .. warning:: raise UserAlreadyExists if user already exists