class_docstr: A ChainMap groups multiple dicts (or other mappings) together
  to create a single, updatable view.
  
  The underlying mappings are stored in a list.  That list is public and can
  be accessed / updated using the *maps* attribute.  There is no other state.
  
  Lookups search the underlying mappings successively until a key is found.
  In contrast, writes, updates, and deletions only operate on the first
  mapping.
code: def __getitem__(self, key):
      for mapping in self.maps:
          try:
              # can't use 'key in mapping' with defaultdict
              return mapping[key]
          except KeyError:
              pass
      # support subclasses that define __missing__
      return self.__missing__(key)
  
imports: import sys
label_desc: Use image node_id 3 for calling the ChainMap obj's underlying member method code with example usage: obj.__getitem__(key) and returns: self, mapping