def update_etag_is_at_header(req, name):
 if (',' in name):
  raise ValueError('Header name must not contain commas')
 existing = req.headers.get('X-Backend-Etag-Is-At')
 req.headers['X-Backend-Etag-Is-At'] = csv_append(existing, name)

def update_etag_is_at_header ( req , name ) : if ( ',' in name ) : raise ValueError ( 'Header name must not contain commas' ) existing = req . headers . get ( 'X-Backend-Etag-Is-At' ) req . headers [ 'X-Backend-Etag-Is-At' ] = csv_append ( existing , name )
helper function to update an x-backend-etag-is-at header whose value is a list of alternative header names at which the actual object etag may be found .