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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 _get_thumbnail_filename(self, source, geometry_string, options):
 """ Computes the destination filename. """

        key = tokey(source.key, geometry_string, serialize(options))
        # make some subdirs
        path = '%s/%s/%s' % (key[:2], key[2:4], key)
        return '%s%s.%s' % (settings.THUMBNAIL_PREFIX, path, EXTENSIONS[options['format']])