code: def lookup_field_class(self, field, obj=None, default=None):
          
          css = ""
  
          # is there a class specified for this field
          if field in self.field_config and 'class' in self.field_config[field]:
              css = self.field_config[field]['class']
  
          # if we were given a default, use that
          elif default:
              css = default
  
          return css
docs: Looks up any additional class we should include when rendering this field