code: def shift(self, periods=1, freq=None, axis=0, fill_value=None):
          
  
          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)
docs: 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