class_docstr: Base class for data series.
  
  .. note::
      This is a base class and should not be used directly.
class_docstr_tok: ["Base","class","for","data","series",".","..","note",":",":","This","is","a","base","class","and","should","not","be","used","directly","."]
docstr: Returns the number of elements in the data series.
docstr_tok: ["Returns","the","number","of","elements","in","the","data","series","."]
code: def __len__(self):
      """Returns the number of elements in the data series."""
      raise NotImplementedError()
  
imports: import abc
  import six
  from six.moves import xrange
  from pyalgotrade import observer
  from pyalgotrade.utils import collections
label_desc: Use image node_id 1 for calling the DataSeries obj's underlying member method code with example usage: obj.__len__() without return types