'Returns the :class:`pyalgotrade.dataseries.bards.BarDataSeries` for a given instrument.
:param instrument: Instrument identifier. If None, the default instrument is returned.
:type instrument: string.
:rtype: :class:`pyalgotrade.dataseries.bards.BarDataSeries`.'

def getDataSeries(self, instrument=None):

    if (instrument is None):
        instrument = self.__defaultInstrument
    return self[instrument]
