class_docstr: Base class of snapshot writers.
  
  :class:`~chainer.training.extensions.Snapshot` invokes ``__call__`` of this
  class every time when taking a snapshot.
  This class determines how the actual saving function will be invoked.
  
  .. seealso::
  
      - :meth:`chainer.training.extensions.snapshot`
code: def _post_save(self):
      for hook in self._post_save_hooks:
          hook()
  
imports: import multiprocessing
  import os
  import shutil
  import threading
  from six.moves import queue
  from chainer.serializers import npz
  from chainer import utils
label_desc: Use image node_id 7 for calling the Writer obj's underlying member method code with example usage: obj._post_save() without return types