'Update the dictionary of attributes used  while rendering the input widget'

def update_widget_attrs(self, bound_field, attrs):

    bound_field.form.update_widget_attrs(bound_field, attrs)
    widget_classes = self.widget.attrs.get(u'class', None)
    if widget_classes:
        if (u'class' in attrs):
            attrs[u'class'] += (u'    ' + widget_classes)
        else:
            attrs.update({u'class': widget_classes})
    return attrs
