code: def to_period(self, freq=None, copy=True):
          
          new_values = self._values
          if copy:
              new_values = new_values.copy()
  
          new_index = self.index.to_period(freq=freq)
          return self._constructor(new_values,
                                   index=new_index).__finalize__(self)
docs: Convert Series from DatetimeIndex to PeriodIndex with desired
          frequency (inferred from index if not passed).
  
          Parameters
          ----------
          freq : str, default None
              Frequency associated with the PeriodIndex.
          copy : bool, default True
              Whether or not to return a copy.
  
          Returns
          -------
          Series
              Series with index converted to PeriodIndex.