def METHOD_NAME(drugbank_id, db_ns):
    """Return a mapping for a DrugBank ID to a given name space.
    Parameters
    ----------
    drugbank_id : str
        DrugBank ID to map.
    db_ns : str
        The database name space to map to.
    Returns
    -------
    str or None
        The ID mapped to the given name space or None if not available.
    """
    return drugbank_to_db.get((drugbank_id, db_ns))