Returns the daily returns for the given period.

        Parameters
        ----------
        start : datetime
            The inclusive starting session label.
        end : datetime, optional
            The inclusive ending session label. If not provided, treat
            ``start`` as a scalar key.

        Returns
        -------
        returns : pd.Series or float
            The returns in the given period. The index will be the trading
            calendar in the range [start, end]. If just ``start`` is provided,
            return the scalar value on that day.

def daily_returns(self, start, end=None):
        """Returns the daily returns for the given period.

        Parameters
        ----------
        start : datetime
            The inclusive starting session label.
        end : datetime, optional
            The inclusive ending session label. If not provided, treat
            ``start`` as a scalar key.

        Returns
        -------
        returns : pd.Series or float
            The returns in the given period. The index will be the trading
            calendar in the range [start, end]. If just ``start`` is provided,
            return the scalar value on that day.
        """
        if end is None:
            return self._daily_returns[start]

        return self._daily_returns[start:end]