def _implementations(cls):
        """Returns all the concrete subclasses of MetadataExtractor.

        """
        if cls.__implementations:
            return cls.__implementations

        cls.__implementations = {}
        for implementation in all_subclasses(MetadataExtractor):
            try:
                feature_name = implementation.feature_name()
                cls.__implementations[feature_name] = implementation
            except NotImplementedError:
                pass
        return cls.__implementations
Returns all the concrete subclasses of MetadataExtractor.