human
I wrote the below code

    if (not isinstance(index_filename, basestring)):
        raise TypeError('Need    a    string    for    the    index    filename')
    if isinstance(filenames, basestring):
        filenames = [filenames]
    if ((filenames is not None) and (not isinstance(filenames, list))):
        raise TypeError('Need    a    list    of    filenames    (as    strings),    or    one    filename')
    if ((format is not None) and (not isinstance(format, basestring))):
        raise TypeError('Need    a    string    for    the    file    format    (lower    case)')
    if (format and (format != format.lower())):
        raise ValueError(("Format    string    '%s'    should    be    lower    case" % format))
    if ((alphabet is not None) and (not (isinstance(alphabet, Alphabet) or isinstance(alphabet, AlphabetEncoder)))):
        raise ValueError(('Invalid    alphabet,    %r' % alphabet))
    from ._index import _FormatToRandomAccess
    from Bio.File import _SQLiteManySeqFilesDict
    repr = ('SeqIO.index_db(%r,    filenames=%r,    format=%r,    alphabet=%r,    key_function=%r)' % (index_filename, filenames, format, alphabet, key_function))
    def proxy_factory(format, filename=None):
        'Given    a    filename    returns    proxy    object,    else    boolean    if    format    OK.'
        if filename:
            return _FormatToRandomAccess[format](filename, format, alphabet)
        else:
            return (format in _FormatToRandomAccess)
    return _SQLiteManySeqFilesDict(index_filename, filenames, proxy_factory, format, key_function, repr)

What's a good function header?
assistant
#Here's a good function header.

def index_db(index_filename, filenames=None, format=None, alphabet=None, key_function=None):