def METHOD_NAME(filename):
    """Guess which db package to use to open a db file.
    Return values:
    - None if the database file can't be read;
    - empty string if the file can be read but can't be recognized
    - the name of the dbm submodule (e.g. "ndbm" or "gnu") if recognized.
    Importing the given module may still fail, and opening the
    database using that module may still fail.
    """
    # Check for ndbm first -- this has a .pag and a .dir file
    try:
        f = io.open(filename + ".pag", "rb")
        f.close()
        f = io.open(filename + ".dir", "rb")
        f.close()
        return "dbm.ndbm"
    except OSError:
        # some dbm emulations based on Berkeley DB generate a .db file
        # some do not, but they should be caught by the bsd checks
        try:
            f = io.open(filename + ".db", "rb")
            f.close()
            # guarantee we can actually open the file using dbm
            # kind of overkill, but since we are dealing with emulations
            # it seems like a prudent step
            if ndbm is not None:
                d = ndbm.open(filename)
                d.close()
                return "dbm.ndbm"
        except OSError:
            pass
    # Check for dumbdbm next -- this has a .dir and a .dat file
    try:
        # First check for presence of files
        os.stat(filename + ".dat")
        size = os.stat(filename + ".dir").st_size
        # dumbdbm files with no keys are empty
        if size == 0:
            return "dbm.dumb"
        f = io.open(filename + ".dir", "rb")
        try:
            if f.read(1) in (b"'", b'"'):
                return "dbm.dumb"
        finally:
            f.close()
    except OSError:
        pass
    # See if the file exists, return None if not
    try:
        f = io.open(filename, "rb")
    except OSError:
        return None
    with f:
        # Read the start of the file -- the magic number
        s16 = f.read(16)
    s = s16[0:4]
    # Return "" if not at least 4 bytes
    if len(s) != 4:
        return ""
    # Convert to 4-byte int in native byte order -- return "" if impossible
    try:
        (magic,) = struct.unpack("=l", s)
    except struct.error:
        return ""
    # Check for GNU dbm
    if magic in (0x13579ace, 0x13579acd, 0x13579acf):
        return "dbm.gnu"
    # Later versions of Berkeley db hash file have a 12-byte pad in
    # front of the file type
    try:
        (magic,) = struct.unpack("=l", s16[-4:])
    except struct.error:
        return ""
    # Unknown
    return ""