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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 fit(self, data=None):
 """ Performs clustering.rst :param data: Data to be fit :return: the clustering.rst object """

        if self.is_pyclustering_instance(self.model):
            if isinstance(self.model, xmeans):
                data = self.input_preprocess(data)
                self.model._xmeans__pointer_data = data
            elif isinstance(self.model, clarans):
                data = self.input_preprocess(data)
                self.model._clarans__pointer_data = data

            self.model.process()
        else:
            self.model.fit(data)
            return self