class_docstr: Base selector implementation.
class_docstr_tok: ["Base","selector","implementation","."]
code: def unregister(self, fileobj):
      try:
          key = self._fd_to_key.pop(self._fileobj_lookup(fileobj))
      except KeyError:
          raise KeyError(
              "{!r} is not registered".format(fileobj)
          ) from None
      return 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 _BaseSelectorImpl obj's underlying member method code with example usage: obj.unregister(fileobj) and returns: key