<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def finish(self):
 """Finishes transconding and returns the video. Returns: bytes Raises: IOError: in case of transcoding error. """

    if self.proc is None:
      return None
    self.proc.stdin.close()
    for thread in (self._out_thread, self._err_thread):
      thread.join()
    (out, err) = [
        b"".join(chunks) for chunks in (self._out_chunks, self._err_chunks)
    ]
    self.proc.stdout.close()
    self.proc.stderr.close()
    if self.proc.returncode:
      err = "\n".join([" ".join(self.cmd), err.decode("utf8")])
      raise IOError(err)
    del self.proc
    self.proc = None
    return out