docs: def save(self, **fields):
          """Save the instance to the remote Transifex server.
  
          If it was pre-populated, it updates the instance on the server,
          otherwise it creates a new object.
  
          Any values given in `fields` will be attempted to be saved
          on the object. The same goes for any other values already set
          to the object by `model_instance.attr = value`.
  
          Raises:
              AttributeError: if a given field is not included in
                  `self.writable_fields`,
          """
          for field in fields:
              if field in self.writable_fields:
                  setattr(self, field, fields[field])
              else:
                  self._handle_wrong_field(field, ATTR_TYPE_WRITE)
  
          if self._populated_fields:
              self._update(**self._modified_fields)
          else:
              self._create(**self._modified_fields)