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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_inline_instances(self, request, obj=None):
 """ Adds RadiusGroupInline only for existing objects """

        inlines = super().get_inline_instances(request, obj)
        if obj:
            usergroup = RadiusUserGroupInline(self.model,
                                              self.admin_site)
            inlines.append(usergroup)
        return inlines