Saving and loading weights
==========================

Lightning can automate saving and loading checkpoints.

Checkpoint saving
-----------------
A Lightning checkpoint has everything needed to restore a training session including:

- 16-bit scaling factor (apex)
- Current epoch
- Global step
- Model state_dict
- State of all optimizers
- State of all learningRate schedulers
- State of all callbacks
- The hyperparameters used for that model if passed in as hparams (Argparse.Namespace)

Automatic saving
^^^^^^^^^^^^^^^^

Checkpointing is enabled by default to the current working directory.
To change the checkpoint path pass in:

.. code-block:: python

    Trainer(default_save_path='/your/path/to/save/checkpoints')

To modify the behavior of checkpointing pass in your own callback.

.. code-block:: python

    from pytorch_lightning.callbacks import ModelCheckpoint

    # DEFAULTS used by the Trainer
    checkpoint_callback = ModelCheckpoint(
        filepath=os.getcwd(),
        save_best_only=True,
        verbose=True,
        monitor='val_loss',
        mode='min',
        prefix=''
    )

    trainer = Trainer(checkpoint_callback=checkpoint_callback)


Or disable it by passing

.. code-block:: python

        trainer = Trainer(checkpoint_callback=False)


The Lightning checkpoint also saves the hparams (hyperparams) passed into the LightningModule init.

.. note:: hparams is a `Namespace <https://docs.python.org/2/library/argparse.html#argparse.Namespace>`_.

.. code-block:: python
   :emphasize-lines: 8

   from argparse import Namespace

   # usually these come from command line args
   args = Namespace(**{'learning_rate':0.001})

   # define you module to have hparams as the first arg
   # this means your checkpoint will have everything that went into making
   # this model (in this case, learning rate)
   class MyLightningModule(pl.LightningModule):

       def __init__(self, hparams, ...):
           self.hparams = hparams

Manual saving
^^^^^^^^^^^^^

To save your own checkpoint call:

.. code-block:: python

   model.save_checkpoint(PATH)

Checkpoint Loading
------------------

You might want to not only load a model but also continue training it. Use this method to
restore the trainer state as well. This will continue from the epoch and global step you last left off.
However, the dataloaders will start from the first batch again (if you shuffled it shouldn't matter).

.. code-block:: python

    model = MyLightingModule.load_from_checkpoint(PATH)
    model.eval()
    y_hat = model(x)

A LightningModule is no different than a nn.Module. This means you can load it and use it for
predictions as you would a nn.Module.