code: def field_for(self, field_id):
          
  
          for field in self.fields:
              if field.id == field_id:
                  return field
          return None
docs: Fetches the field for the given Field ID.
  
          :param field_id: ID for Field to fetch.
          :return: :class:`ContentTypeField <ContentTypeField>` object.
          :rtype: contentful.ContentTypeField