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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 group_by(self, *args):
 """ This method lets you specify the grouping fields explicitly. The `args` must be names of grouping fields or calculated fields that this queryset was created with. """

        for name in args:
            assert name in self._fields or name in self._calculated_fields, \
                   'Cannot group by `%s` since it is not included in the query' % name
        qs = copy(self)
        qs._grouping_fields = args
        return qs