class_docstr: A base class for common vocabulary operations. Each subclass should at least 
  implement its own build_vocab() function.
code: def unit2id(self, unit):
      unit = self.normalize_unit(unit)
      if unit in self._unit2id:
          return self._unit2id[unit]
      else:
          return self._unit2id[UNK]
  
imports: from copy import copy
  from collections import Counter, OrderedDict
  from collections.abc import Iterable
  import os
  import pickle
label_desc: Use image node_id 6 for calling the BaseVocab obj's underlying member method code with example usage: obj.unit2id(unit) and returns: self, self