code: def _fields_list_to_dict(fields, option_name):
      
      if isinstance(fields, collections.Mapping):
          return fields
  
      if isinstance(fields, collections.Sequence):
          if not all(isinstance(field, string_type) for field in fields):
              raise TypeError("%s must be a list of key names, each an "
                              "instance of %s" % (option_name,
                                                  string_type.__name__))
          return dict.fromkeys(fields, 1)
  
      raise TypeError("%s must be a mapping or "
                      "list of key names" % (option_name,))
docs: Takes a sequence of field names and returns a matching dictionary.
  
      ["a", "b"] becomes {"a": 1, "b": 1}
  
      and
  
      ["a.b.c", "d", "a.c"] becomes {"a.b.c": 1, "d": 1, "a.c": 1}