63 lines
2.5 KiB
ReStructuredText
63 lines
2.5 KiB
ReStructuredText
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*******************
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Save Callback state
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*******************
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Some callbacks require internal state in order to function properly. You can optionally
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choose to persist your callback's state as part of model checkpoint files using
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:meth:`~pytorch_lightning.callbacks.Callback.state_dict` and :meth:`~pytorch_lightning.callbacks.Callback.load_state_dict`.
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Note that the returned state must be able to be pickled.
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When your callback is meant to be used only as a singleton callback then implementing the above two hooks is enough
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to persist state effectively. However, if passing multiple instances of the callback to the Trainer is supported, then
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the callback must define a :attr:`~pytorch_lightning.callbacks.Callback.state_key` property in order for Lightning
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to be able to distinguish the different states when loading the callback state. This concept is best illustrated by
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the following example.
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.. testcode::
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class Counter(Callback):
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def __init__(self, what="epochs", verbose=True):
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self.what = what
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self.verbose = verbose
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self.state = {"epochs": 0, "batches": 0}
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@property
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def state_key(self):
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# note: we do not include `verbose` here on purpose
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return self._generate_state_key(what=self.what)
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def on_train_epoch_end(self, *args, **kwargs):
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if self.what == "epochs":
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self.state["epochs"] += 1
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def on_train_batch_end(self, *args, **kwargs):
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if self.what == "batches":
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self.state["batches"] += 1
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def load_state_dict(self, state_dict):
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self.state.update(state_dict)
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def state_dict(self):
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return self.state.copy()
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# two callbacks of the same type are being used
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trainer = Trainer(callbacks=[Counter(what="epochs"), Counter(what="batches")])
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A Lightning checkpoint from this Trainer with the two stateful callbacks will include the following information:
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.. code-block::
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{
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"state_dict": ...,
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"callbacks": {
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"Counter{'what': 'batches'}": {"batches": 32, "epochs": 0},
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"Counter{'what': 'epochs'}": {"batches": 0, "epochs": 2},
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...
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}
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}
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The implementation of a :attr:`~pytorch_lightning.callbacks.Callback.state_key` is essential here. If it were missing,
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Lightning would not be able to disambiguate the state for these two callbacks, and :attr:`~pytorch_lightning.callbacks.Callback.state_key`
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by default only defines the class name as the key, e.g., here ``Counter``.
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