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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 shift(self, periods=1, freq=None, axis=0, fill_value=None):
 """ Shift each group by periods observations. Parameters periods : integer, default 1 number of periods to shift freq : frequency string axis : axis to shift, default 0 fill_value : optional .. versionadded:: 0.24.0 """


        if freq is not None or axis != 0 or not isna(fill_value):
            return self.apply(lambda x: x.shift(periods, freq,
                                                axis, fill_value))

        return self._get_cythonized_result('group_shift_indexer',
                                           self.grouper, cython_dtype=np.int64,
                                           needs_ngroups=True,
                                           result_is_index=True,
                                           periods=periods)