def METHOD_NAME(self) -> dict[str, str]:
    """Pre-populate a key-value pair of persistence data (if empty).
    Example of pre-populated mapping:
    ```py
    {
        "default": "sql",
        "user": "spanner",
        "site": "sql",
        "cache": "sql",
        "token": "sql",
        "session": "sql",
    }
    ```
    """
    if not self._mapping:
        if self.type != "hybrid":
            self._mapping = dict.fromkeys(PERSISTENCE_DATA_KEYS, self.type)
        else:
            self._mapping = self.validate_hybrid_mapping()
    return self._mapping