Cast to PeriodArray/Index at a particular frequency.

        Converts DatetimeArray/Index to PeriodArray/Index.

        Parameters
        ----------
        freq : str or Offset, optional
            One of pandas' :ref:`offset strings <timeseries.offset_aliases>`
            or an Offset object. Will be inferred by default.

        Returns
        -------
        PeriodArray/Index

        Raises
        ------
        ValueError
            When converting a DatetimeArray/Index with non-regular values,
            so that a frequency cannot be inferred.

        See Also
        --------
        PeriodIndex: Immutable ndarray holding ordinal values.
        DatetimeIndex.to_pydatetime: Return DatetimeIndex as object.

        Examples
        --------
        >>> df = pd.DataFrame({"y": [1, 2, 3]},
        ...                   index=pd.to_datetime(["2000-03-31 00:00:00",
        ...                                         "2000-05-31 00:00:00",
        ...                                         "2000-08-31 00:00:00"]))
        >>> df.index.to_period("M")
        PeriodIndex(['2000-03', '2000-05', '2000-08'],
                    dtype='period[M]', freq='M')

        Infer the daily frequency

        >>> idx = pd.date_range("2017-01-01", periods=2)
        >>> idx.to_period()
        PeriodIndex(['2017-01-01', '2017-01-02'],
                    dtype='period[D]', freq='D')

def to_period(self, freq=None):
        """
        Cast to PeriodArray/Index at a particular frequency.

        Converts DatetimeArray/Index to PeriodArray/Index.

        Parameters
        ----------
        freq : str or Offset, optional
            One of pandas' :ref:`offset strings <timeseries.offset_aliases>`
            or an Offset object. Will be inferred by default.

        Returns
        -------
        PeriodArray/Index

        Raises
        ------
        ValueError
            When converting a DatetimeArray/Index with non-regular values,
            so that a frequency cannot be inferred.

        See Also
        --------
        PeriodIndex: Immutable ndarray holding ordinal values.
        DatetimeIndex.to_pydatetime: Return DatetimeIndex as object.

        Examples
        --------
        >>> df = pd.DataFrame({"y": [1, 2, 3]},
        ...                   index=pd.to_datetime(["2000-03-31 00:00:00",
        ...                                         "2000-05-31 00:00:00",
        ...                                         "2000-08-31 00:00:00"]))
        >>> df.index.to_period("M")
        PeriodIndex(['2000-03', '2000-05', '2000-08'],
                    dtype='period[M]', freq='M')

        Infer the daily frequency

        >>> idx = pd.date_range("2017-01-01", periods=2)
        >>> idx.to_period()
        PeriodIndex(['2017-01-01', '2017-01-02'],
                    dtype='period[D]', freq='D')
        """
        from pandas.core.arrays import PeriodArray

        if self.tz is not None:
            warnings.warn("Converting to PeriodArray/Index representation "
                          "will drop timezone information.", UserWarning)

        if freq is None:
            freq = self.freqstr or self.inferred_freq

            if freq is None:
                raise ValueError("You must pass a freq argument as "
                                 "current index has none.")

            freq = get_period_alias(freq)

        return PeriodArray._from_datetime64(self._data, freq, tz=self.tz)