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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_ordering(self):
 """ Returns ordering value for list. :rtype: str. """

        #noinspection PyUnresolvedReferences
        ordering = self.request.GET.get('ordering', None)

        if ordering not in ['title', '-created_at']:
            ordering = '-created_at'

        return ordering