code: def unregister(self, fileobj):
          
          try:
              key = self._fd_to_key.pop(self._fileobj_lookup(fileobj))
          except KeyError:
              raise KeyError("{0!r} is not registered".format(fileobj))
  
          # Getting the fileno of a closed socket on Windows errors with EBADF.
          except socket.error as err:
              if err.errno != errno.EBADF:
                  raise
              else:
                  for key in self._fd_to_key.values():
                      if key.fileobj is fileobj:
                          self._fd_to_key.pop(key.fd)
                          break
                  else:
                      raise KeyError("{0!r} is not registered".format(fileobj))
          return key
docs: Unregister a file object from being monitored.