def _read_schema(self):
        """return the local filename of the definition file for this schema
        if not present or older than expiry, pull the latest version from
        the web at self._ontology_file"""
        cache_filename = os.path.join(
            CACHE_ROOT, "%s.smt" % self._representation)
        log.info("Attempting to read local schema at %s" % cache_filename)
        try:
            if time.time() - os.stat(cache_filename).st_mtime > CACHE_EXPIRY:
                log.warning("Cache expired, re-pulling")
                self._pull_schema_definition(cache_filename)
        except OSError:
            log.warning("Local schema not found. Pulling from web.")
            self._pull_schema_definition(cache_filename)
        else:
            log.info("Success")

        return cache_filename
return the local filename of the definition file for this schema
        if not present or older than expiry, pull the latest version from
        the web at self._ontology_file