class_docstr: Evaluation mixin class.
  
  This class has a `name` attribute which is used for display purposes and
  defaults to 'None'.
  
  `METRIC_NAMES` is a list of tuples, containing the attribute's name and the
  corresponding label, e.g.:
  
  The attributes defined in `METRIC_NAMES` will be provided as an ordered
  dictionary as the `metrics` property unless the subclass overwrites the
  property.
  
  `FLOAT_FORMAT` is used to format floats.
class_docstr_tok: ["Evaluation","mixin","class",".","This","class","has","a","`","name","`","attribute","which","is","used","for","display","purposes","and","defaults","to","'None","'",".","`","METRIC_NAMES","`","is","a","list","of","tuples",",","containing","the","attribute","'s","name","and","the","corresponding","label",",","e.g",".",":","The","attributes","defined","in","`","METRIC_NAMES","`","will","be","provided","as","an","ordered","dictionary","as","the","`","metrics","`","property","unless","the","subclass","overwrites","the","property",".","`","FLOAT_FORMAT","`","is","used","to","format","floats","."]
docstr: Length of the evaluation object.
docstr_tok: ["Length","of","the","evaluation","object","."]
code: def __len__(self):
      """Length of the evaluation object."""
      raise NotImplementedError("must be implemented by subclass.")
  
imports: from __future__ import absolute_import, division, print_function
  import numpy
  from .None import chords, beats, notes, onsets, tempo
  from .beats import BeatEvaluation, BeatMeanEvaluation
  from .chords import ChordEvaluation, ChordMeanEvaluation, ChordSumEvaluation
  from .key import KeyEvaluation, KeyMeanEvaluation
  from .notes import NoteEvaluation, NoteMeanEvaluation, NoteSumEvaluation
  from .onsets import OnsetEvaluation, OnsetMeanEvaluation, OnsetSumEvaluation
  from .tempo import TempoEvaluation, TempoMeanEvaluation
label_desc: Use image node_id 2 for calling the EvaluationMixin obj's underlying member method code with example usage: obj.__len__() without return types