'Updated the widget attributes which shall be added to the widget when rendering this field.'

def update_widget_attrs(self, bound_field, attrs):

    if (bound_field.field.has_subwidgets() is False):
        widget_classes = getattr(self, u'widget_css_classes', None)
        if widget_classes:
            if (u'class' in attrs):
                attrs[u'class'] += (u'    ' + widget_classes)
            else:
                attrs.update({u'class': widget_classes})
    return attrs
