Callbacks enable you, or the users of your code, to add new behavior to the training loop without needing to modify the source code.

Add a callback interface to your loop

Suppose we want to enable anyone to run some arbitrary code at the end of a training iteration. Here is how that gets done in Fabric:
class MyCallback:
    def on_train_batch_end(self, loss, output):
        # Here, put any code you want to run at the end of a training step
from lightning.fabric import Fabric

# The code of a callback can live anywhere, away from the training loop
from my_callbacks import MyCallback

# Add one or several callbacks:
fabric = Fabric(callbacks=[MyCallback()])


for iteration, batch in enumerate(train_dataloader):

    # Let a callback add some arbitrary processing at the appropriate place
    # Give the callback access to some varibles"on_train_batch_end", loss=loss, output=...)

As you can see, the code inside the callback method is completely decoupled from the trainer code. This enables flexibility in extending the loop in arbitrary ways.

Exercise: Implement a callback that computes and prints the time to complete an iteration.

Multiple callbacks

The callback system is designed to easily run multiple callbacks at the same time. You can pass a list to Fabric:

# Add multiple callback implementations in a list
callback1 = LearningRateMonitor()
callback2 = Profiler()
fabric = Fabric(callbacks=[callback1, callback2])

# Let Fabric call the implementations (if they exist)"any_callback_method", arg1=..., arg2=...)

# is the same as doing this
callback1.any_callback_method(arg1=..., arg2=...)
callback2.any_callback_method(arg1=..., arg2=...)

The call() calls the callback objects in the order they were given to Fabric. Not all objects registered via Fabric(callbacks=...) must implement a method with the given name. The ones that have a matching method name will get called.

Next steps

Callbacks are a powerful tool for building a Trainer. See a real example of how they can be integrated in our Trainer template based on Fabric: