class_docstr: Expression base class
  
  Expression is designed to handle the calculation of data with the format below
  data with two dimension for each instrument,
  
  - feature
  - time:  it  could be observation time or period time.
  
      - period time is designed for Point-in-time database.  For example, the period time maybe 2014Q4, its value can observed for multiple times(different value may be observed at different time due to amendment).
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docstr: Get the longest length of historical data the feature has accessed
  
  This is designed for getting the needed range of the data to calculate
  the features in specific range at first.  However, situations like
  Ref(Ref($close, -1), 1) can not be handled rightly.
  
  So this will only used for detecting the length of historical data needed.
docstr_tok: ["Get","the","longest","length","of","historical","data","the","feature","has","accessed","This","is","designed","for","getting","the","needed","range","of","the","data","to","calculate","the","features","in","specific","range","at","first",".","However",",","situations","like","Ref","(","Ref","(","$","close",",","-1",")",",","1",")","can","not","be","handled","rightly",".","So","this","will","only","used","for","detecting","the","length","of","historical","data","needed","."]
code: def get_longest_back_rolling(self):
      """Get the longest length of historical data the feature has accessed
  
      This is designed for getting the needed range of the data to calculate
      the features in specific range at first.  However, situations like
      Ref(Ref($close, -1), 1) can not be handled rightly.
  
      So this will only used for detecting the length of historical data needed.
      """
      # TODO: forward operator like Ref($close, -1) is not supported yet.
      raise NotImplementedError(
          "This function must be implemented in your newly defined feature"
      )
  
imports: from __future__ import division
  from __future__ import print_function
  import abc
  import pandas
  from ..log import get_module_logger
label_desc: Use image node_id 27 for calling the Expression obj's underlying member method code with example usage: obj.get_longest_back_rolling() without return types