@classmethod<EOL><INDENT>def _get_extension_classes(cls):<DEDENT>
if cls._extension_classes is None:<EOL><INDENT>exts = {}<EOL>for ext in entry.points[NAMESPACE_EXTENSIONS]:<EOL><INDENT>exts.setdefault(ext.priority, [])<EOL>exts[ext.priority].append(ext)<EOL><DEDENT>cls._extension_classes = list(utils.iter_prio_dict(exts))<EOL><DEDENT>return cls._extension_classes<EOL>
Retrieve the extension classes in priority order.

:returns: A list of extension classes, in proper priority
          order.