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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _filenames_from_arg(filename):
 """Utility function to deal with polymorphic filenames argument."""

    if isinstance(filename, string_types):
        filenames = [filename]
    elif isinstance(filename, (list, tuple)):
        filenames = filename
    else:
        raise Exception('filename argument must be string, list or tuple')
    for fn in filenames:
        if not os.path.exists(fn):
            raise ValueError('file not found: %s' % fn)
        if not os.path.isfile(fn):
            raise ValueError('not a file: %s' % fn)
    return filenames