class_docstr: A base class for common vocabulary operations. Each subclass should at least 
  implement its own build_vocab() function.
code: def __getitem__(self, key):
      if isinstance(key, str):
          return self.unit2id(key)
      elif isinstance(key, int) or isinstance(key, list):
          return self.id2unit(key)
      else:
          raise TypeError("Vocab key must be one of str, list, or int")
  
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 11 for calling the BaseVocab obj's underlying member method code with example usage: obj.__getitem__(key) and returns: self, self