code: def update_fields(self, updates):
          
  
          for field in updates:
              if self._valid_field(field):
                  self._track_changes.add(field)
              else:
                  raise ValueError('"{}" is not a valid internal field name'.format(field))
  
          # Update existing instance of fields, or create a fields instance if needed
          if self.fields:
              self.fields.update(updates)
          else:
              self.fields = updates
docs: Update the value for a field(s) in the listitem
  
          :param update: A dict of {'field name': newvalue}