code: def update_widget_attrs(self, bound_field, attrs):
          
          if bound_field.field.has_subwidgets() is False:
              widget_classes = getattr(self, 'widget_css_classes', None)
              if widget_classes:
                  if 'class' in attrs:
                      attrs['class'] += ' ' + widget_classes
                  else:
                      attrs.update({'class': widget_classes})
          return attrs
docs: Updated the widget attributes which shall be added to the widget when rendering this field.