class_docstr: Wraps a resolver with a mapping of overrides.
  
  This can be used to make local DNS changes (e.g. for testing)
  without modifying system-wide settings.
  
  The mapping can contain either host strings or host-port pairs.
code: def initialize(self, resolver, mapping):
      self.resolver = resolver
      self.mapping = mapping
  
imports: from __future__ import absolute_import, division, print_function
  import errno
  import os
  import sys
  import socket
  import stat
  from tornado.concurrent import dummy_executor, run_on_executor
  from tornado.ioloop import IOLoop
  from tornado.platform.auto import set_close_exec
  from tornado.util import PY3, Configurable, errno_from_exception
label_desc: Use image node_id 1 for calling the OverrideResolver obj's underlying member method code with example usage: obj.initialize(resolver, mapping) without return types