class_docstr: A ChainMap groups multiple dicts (or other mappings) together
  to create a single, updateable view.
  
  The underlying mappings are stored in a list.  That list is public and can
  accessed or 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.
class_docstr_tok: ["A","ChainMap","groups","multiple","dicts","(","or","other","mappings",")","together","to","create","a","single",",","updateable","view",".","The","underlying","mappings","are","stored","in","a","list",".","That","list","is","public","and","can","accessed","or","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: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from __future__ import unicode_literals
  import abc
  import os
  from io import open
  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