'Convert an id to a Content-ID header value.
Args:
id_: string, identifier of individual request.
Returns:
A Content-ID header with the id_ encoded into it. A UUID is prepended to
the value because Content-ID headers are supposed to be universally
unique.'

def _id_to_header(self, id_):

    if (self._base_id is None):
        self._base_id = uuid.uuid4()
    return ('<%s+%s>' % (self._base_id, quote(id_)))
