class_docstr: Base class for :class:`pyalgotrade.bar.Bar` providing feeds.
  
  :param frequency: The bars frequency. Valid values defined in :class:`pyalgotrade.bar.Frequency`.
  :param maxLen: The maximum number of values that the :class:`pyalgotrade.dataseries.bards.BarDataSeries` will hold.
      Once a bounded length is full, when new items are added, a corresponding number of items are discarded
      from the opposite end. If None then dataseries.DEFAULT_MAX_LEN is used.
  :type maxLen: int.
  
  .. note::
      This is a base class and should not be used directly.
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code: def createDataSeries(self, key, maxLen):
      ret = bards.BarDataSeries(maxLen)
      ret.setUseAdjustedValues(self.__useAdjustedValues)
      return ret
  
imports: import abc
  from pyalgotrade import bar
  from pyalgotrade.dataseries import bards
  from pyalgotrade import feed
  from pyalgotrade import dispatchprio
label_desc: Use image node_id 7 for calling the BaseBarFeed obj's underlying member method code with example usage: obj.createDataSeries(key, maxLen) and returns: ret