class_docstr: Mapping of file objects to selector keys.
class_docstr_tok: ["Mapping","of","file","objects","to","selector","keys","."]
code: def __iter__(self):
      return iter(self._selector._fd_to_key)
  
imports: from abc import ABCMeta, abstractmethod
  from collections import namedtuple
  from collections.abc import Mapping
  import math
  import select
  import sys
label_desc: Use image node_id 4 for calling the _SelectorMapping obj's underlying member method code with example usage: obj.__iter__() and returns: iter