Save Callback state

Some callbacks require internal state in order to function properly. You can optionally choose to persist your callback’s state as part of model checkpoint files using state_dict() and load_state_dict(). Note that the returned state must be able to be pickled.

When your callback is meant to be used only as a singleton callback then implementing the above two hooks is enough to persist state effectively. However, if passing multiple instances of the callback to the Trainer is supported, then the callback must define a state_key property in order for Lightning to be able to distinguish the different states when loading the callback state. This concept is best illustrated by the following example.

class Counter(Callback):
    def __init__(self, what="epochs", verbose=True):
        self.what = what
        self.verbose = verbose
        self.state = {"epochs": 0, "batches": 0}

    def state_key(self) -> str:
        # note: we do not include `verbose` here on purpose
        return f"Counter[what={self.what}]"

    def on_train_epoch_end(self, *args, **kwargs):
        if self.what == "epochs":
            self.state["epochs"] += 1

    def on_train_batch_end(self, *args, **kwargs):
        if self.what == "batches":
            self.state["batches"] += 1

    def load_state_dict(self, state_dict):

    def state_dict(self):
        return self.state.copy()

# two callbacks of the same type are being used
trainer = Trainer(callbacks=[Counter(what="epochs"), Counter(what="batches")])

A Lightning checkpoint from this Trainer with the two stateful callbacks will include the following information:

    "state_dict": ...,
    "callbacks": {
        "Counter{'what': 'batches'}": {"batches": 32, "epochs": 0},
        "Counter{'what': 'epochs'}": {"batches": 0, "epochs": 2},

The implementation of a state_key is essential here. If it were missing, Lightning would not be able to disambiguate the state for these two callbacks, and state_key by default only defines the class name as the key, e.g., here Counter.