
## Write a callback

The places where each method gets called is demonstrated in this snippet:

```python
def main_loop():
  # create graph for the model
  callbacks.setup_graph()
  # create session, initialize session, finalize graph ...
  # start training:
  callbacks.before_train()
  for epoch in range(epoch_start, epoch_end):
    callbacks.before_epoch()
    for step in range(steps_per_epoch):
      run_step()  # callbacks.{before,after}_run are hooked with session
      callbacks.trigger_step()
    callbacks.after_epoch()
    callbacks.trigger_epoch()
  callbacks.after_train()
```

## Explain the callback methods

You can overwrite any of the following methods to define a new callback:

* `_setup_graph(self)`

Setup the ops / tensors in the graph which you might need to use in the callback. You can use
[`graph.get_tensor_by_name`](https://www.tensorflow.org/api_docs/python/tf/Graph#get_tensor_by_name)
to access those already defined in the training tower. Or use
[`self.trainer.get_predictor(..)`](http://tensorpack.readthedocs.io/en/latest/modules/train.html?highlight=get_predictor#tensorpack.train.Trainer.get_predictor)
to create a callable evaluation function in the predict tower.

This method is to separate between "define" and "run", and also to avoid the common mistake to create ops inside
loops, all changes to the graph should be made in this method.

* `_before_train(self)`

Can be used to run some manual initialization of variables, or start some services for the training.

* `_after_train(self)`

Usually some finalization work.

* `_before_epoch(self)`, `_after_epoch(self)`

Use them only when you really need something to happen __immediately__ before/after an epoch.
Otherwise, `_trigger_epoch` should be enough.

* `_before_run(self, ctx)`, `_after_run(self, ctx, values)`

This two are the equivlent of [tf.train.SessionRunHook](https://www.tensorflow.org/api_docs/python/tf/train/SessionRunHook).
Please refer to TensorFlow documentation for detailed API.
They are used to run extra ops / eval extra tensors / feed extra values __along with__ the actual training iteration.

Note the difference between running __along with__ an iteration and running after an iteration.
When you write

```python
def _before_run(self, _):
  return tf.train.SessionRunArgs(fetches=my_op)
```

The training loops would become `sess.run([training_op, my_op])`.
This is different from `sess.run(training_op); sess.run(my_op);`,
which is what you would get if you run the op in `_trigger_step`.

* `_trigger_step(self)`

Do something (including running ops) after each step has finished.
Be careful to only do light work here because it could affect training speed.

* `_trigger_epoch(self)`

Do something after each epoch has finished. Will call `self.trigger()` by default.

* `_trigger(self)`

By default will get called by `_trigger_epoch`,
but you can customize the scheduling of this callback by
`PeriodicTrigger`, to let this method run every k steps or every k epochs.

## What you can do in the callback

* Access tensors / ops in either training / inference mode (need to create them in `_setup_graph`).
* Write stuff to the monitor backend, by `self.trainer.monitors.put_xxx`.
	The monitors might direct your events to TensorFlow events file, JSON file, stdout, etc.
	You can get history monitor data as well. See the docs for [Monitors](http://tensorpack.readthedocs.io/en/latest/modules/callbacks.html#tensorpack.callbacks.Monitors)
* Access the current status of training, such as `epoch_num`, `global_step`. See [here](http://tensorpack.readthedocs.io/en/latest/modules/callbacks.html#tensorpack.callbacks.Callback)
* Anything else that can be done with plain python.
