class_docstr: Base class for "value getters" (e.g., class DictValueGetter) that are used
  to resolve values of sub-fields after the experiment's config dictionary (in
  description.py) is defined and possibly updated from a sub-experiment.
  
  This solves the problem of referencing the config dictionary's field from within
  the definition of the dictionary itself (before the dictionary's own defintion
  is complete).
  
  NOTE: its possible that the referenced value does not yet exist at the
        time of instantiation of a given value-getter future. It will be
        resolved when the base description.py calls
        applyValueGettersToContainer().
  
  NOTE: The constructor of the derived classes MUST call our constructor.
  NOTE: The derived classes MUST override handleGetValue(self).
  
  NOTE: may be used by base and sub-experiments to derive their own custom value
    getters; however, their use is applicapble only where permitted, as
    described in comments within descriptionTemplate.tpl. See class
    DictValueGetter for implementation example.
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code: def __init__(self):
      # print("NOTE: ValueGetterBase INITIALIZING")
      self.__inLookup = False
      self.__cachedResult = self.__NoResult
  
file_path: nupic-legacy/src/nupic/frameworks/opf/exp_description_helpers.py
imports: import os
  import imp
  from nupic.data.dict_utils import rUpdate
label_desc: Use image node_id 1 to create a new ValueGetterBase object from inherited base classes: object with example: obj = ValueGetterBase()