def _clear_fields(cls, collection_name, field_names):
        """Clears the values for the given fields (i.e., sets them to None)
        on all in-memory frames in the collection.
        """
        if collection_name not in cls._instances:
            return

        for frames in cls._instances[collection_name].values():
            for frame in frames.values():
                for field_name in field_names:
                    frame._doc._data[field_name] = None
Clears the values for the given fields (i.e., sets them to None)
        on all in-memory frames in the collection.
        
fiftyone/core/singletons.py