code: def daily_returns(self, start, end=None):
          
          if end is None:
              return self._daily_returns[start]
  
          return self._daily_returns[start:end]
docs: 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.