def add_custom_splits(
        self, string: str, custom_splits: Iterable[CustomSplit]
    ) -> None:
        """Custom Split Map Setter Method

        Side Effects:
            Adds a mapping from @string to the custom splits @custom_splits.
        """
        key = self._get_key(string)
        self._CUSTOM_SPLIT_MAP[key] = tuple(custom_splits)
Custom Split Map Setter Method

        Side Effects:
            Adds a mapping from @string to the custom splits @custom_splits.
        
src/black/trans.py