# Callbacks¶

Apart from the actual training iterations that minimize the cost, you almost surely would like to do something else. Callbacks are such an interface to describe what to do besides the training iterations.

There are several places where you might want to do something else:

• Before the training has started (e.g. initialize the saver, dump the graph)

• Along with each training iteration (e.g. run some other operations in the graph)

• Between training iterations (e.g. update the progress bar, update hyperparameters)

• Between epochs (e.g. save the model, run some validation)

• After the training (e.g. send the model somewhere, send a message to your phone)

We found people traditionally tend to write the training loop together with these extra features. This makes the loop lengthy, and the code for the same feature probably get separated. By writing callbacks to implement what to do at each place, tensorpack trainers will call the callbacks at the proper time. Therefore the code can be reused with one single line, as long as you are using tensorpack trainers.

For example, these are the callbacks I used when training a ResNet:

TrainConfig(
# ...
callbacks=[
# save the model every epoch
ModelSaver(),
# backup the model with best validation error
MinSaver('val-error-top1'),
# run inference on another Dataflow every epoch, compute top1/top5 classification error and save them in log
InferenceRunner(dataset_val, [
ClassificationError('wrong-top1', 'val-error-top1'),
ClassificationError('wrong-top5', 'val-error-top5')]),
# schedule the learning rate based on epoch number
ScheduledHyperParamSetter('learning_rate',
[(30, 1e-2), (60, 1e-3), (85, 1e-4), (95, 1e-5)]),
# allow manually setting the learning rate during training
HumanHyperParamSetter('learning_rate'),
# send validation error to my phone through pushbullet
SendStat('curl -u your_id_xxx: https://api.pushbullet.com/v2/pushes \\
-d type=note -d title="validation error" \\
-d body={val-error-top1} > /dev/null 2>&1',
'val-error-top1')
],
extra_callbacks=[    # these callbacks are enabled by default already
# maintain and summarize moving average of some tensors defined in the model (e.g. training loss, training error)
MovingAverageSummary(),
# draw a nice progress bar
ProgressBar(),
# run tf.summary.merge_all every epoch and send results to monitors
MergeAllSummaries(),
# run ops in GraphKeys.UPDATE_OPS collection along with training, if any
RunUpdateOps(),
],
monitors=[        # monitors are a special kind of callbacks. these are also enabled by default
# write everything to tensorboard
TFEventWriter(),
# write all scalar data to a json file, for easy parsing
JSONWriter(),
# print all scalar data every epoch (can be configured differently)
ScalarPrinter(),
]
)


Notice that callbacks cover every detail of training, ranging from graph operations to the progress bar. This means you can customize every part of the training to your preference, e.g. display something different in the progress bar, evaluating part of the summaries at a different frequency, etc.

See Write a callback on how to implement a callback.