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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 _set_dtype(self, dtype):
 """ Internal method for directly updating the CategoricalDtype Parameters dtype : CategoricalDtype Notes ----- We don't do any validation here. It's assumed that the dtype is a (valid) instance of `CategoricalDtype`. """

        codes = _recode_for_categories(self.codes, self.categories,
                                       dtype.categories)
        return type(self)(codes, dtype=dtype, fastpath=True)