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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 config_string(self):
 """ Build the storable string corresponding to this configuration object. :rtype: string """

        return (gc.CONFIG_STRING_SEPARATOR_SYMBOL).join(
            [u"%s%s%s" % (fn, gc.CONFIG_STRING_ASSIGNMENT_SYMBOL, self.data[fn]) for fn in sorted(self.data.keys()) if self.data[fn] is not None]
        )