, filename='snapshot_iter_{.updater.iteration}', *, target=None, condition=None, writer=None, snapshot_on_error=False)[source]

Returns a trainer extension to take snapshots of the trainer.

This extension serializes the trainer object and saves it to the output directory. It is used to support resuming the training loop from the saved state.

This extension is called once per epoch by default. To take a snapshot at a different interval, a trigger object specifying the required interval can be passed along with this extension to the extend() method of the trainer.

The default priority is -100, which is lower than that of most built-in extensions.


This extension first writes the serialized object to a temporary file and then rename it to the target file name. Thus, if the program stops right before the renaming, the temporary file might be left in the output directory.

  • savefun – Function to save the trainer. It takes two arguments: the output file path and the trainer object. It is chainer.serializers.save_npz() by default. If writer is specified, this argument must be None.

  • filename (str) – Name of the file into which the trainer is serialized. It can be a format string, where the trainer object is passed to the str.format() method.

  • target – Object to serialize. If it is not specified, it will be the trainer object.

  • condition – Condition object. It must be a callable object that returns boolean without any arguments. If it returns True, the snapshot will be done. If not, it will be skipped. The default is a function that always returns True.

  • writer – Writer object. It must be a callable object. See below for the list of built-in writers. If savefun is other than None, this argument must be None. In that case, a SimpleWriter object instantiated with specified savefun argument will be used.

  • snapshot_on_error (bool) – Whether to take a snapshot in case trainer loop has been failed.


Snapshot extension object.

Using asynchronous writers

By specifying writer argument, writing operations can be made asynchronous, hiding I/O overhead of snapshots.

>>> from import extensions
>>> writer = extensions.snapshot_writers.ProcessWriter()
>>> trainer.extend(extensions.snapshot(writer=writer), trigger=(1, 'epoch'))

To change the format, such as npz or hdf5, you can pass a saving function as savefun argument of the writer.

>>> from import extensions
>>> from chainer import serializers
>>> writer = extensions.snapshot_writers.ProcessWriter(
...     savefun=serializers.save_npz)
>>> trainer.extend(extensions.snapshot(writer=writer), trigger=(1, 'epoch'))

This is the list of built-in snapshot writers.