LightningModule

class lightning.pytorch.core.LightningModule(*args, **kwargs)

Bases: _DeviceDtypeModuleMixin, HyperparametersMixin, ModelHooks, DataHooks, CheckpointHooks, Module

all_gather(data, group=None, sync_grads=False)

Gather tensors or collections of tensors from multiple processes.

This method needs to be called on all processes. Failing to do so will cause your program to stall forever.

Parameters

• data (Union[Tensor, Dict, List, Tuple]) – int, float, tensor of shape (batch, …), or a (possibly nested) collection thereof.

• group (Optional[Any]) – the process group to gather results from. Defaults to all processes (world)

• sync_grads (bool) – flag that allows users to synchronize gradients for the all_gather operation

Return type

Union[Tensor, Dict, List, Tuple]

Returns

A tensor of shape (world_size, batch, …), or if the input was a collection the output will also be a collection with tensors of this shape.

backward(loss, *args, **kwargs)

Called to perform backward on the loss returned in training_step(). Override this hook with your own implementation if you need to.

Parameters

loss (Tensor) – The loss tensor returned by training_step(). If gradient accumulation is used, the loss here holds the normalized value (scaled by 1 / accumulation steps).

Example:

def backward(self, loss):
    loss.backward()

Return type

None

clip_gradients(optimizer, gradient_clip_val=None, gradient_clip_algorithm=None)

Handles gradient clipping internally.

Note

• Do not override this method. If you want to customize gradient clipping, consider using configure_gradient_clipping() method.

• For manual optimization (self.automatic_optimization = False), if you want to use gradient clipping, consider calling self.clip_gradients(opt, gradient_clip_val=0.5, gradient_clip_algorithm="norm") manually in the training step.

Parameters

• optimizer (Optimizer) – Current optimizer being used.

• gradient_clip_val (Union[int, float, None]) – The value at which to clip gradients.

• gradient_clip_algorithm (Optional[str]) – The gradient clipping algorithm to use. Pass gradient_clip_algorithm="value" to clip by value, and gradient_clip_algorithm="norm" to clip by norm.

Return type

None

configure_callbacks()

Configure model-specific callbacks. When the model gets attached, e.g., when .fit() or .test() gets called, the list or a callback returned here will be merged with the list of callbacks passed to the Trainer’s callbacks argument. If a callback returned here has the same type as one or several callbacks already present in the Trainer’s callbacks list, it will take priority and replace them. In addition, Lightning will make sure ModelCheckpoint callbacks run last.

Return type

Union[Sequence[Callback], Callback]

Returns

A callback or a list of callbacks which will extend the list of callbacks in the Trainer.

Example:

def configure_callbacks(self):
    early_stop = EarlyStopping(monitor="val_acc", mode="max")
    checkpoint = ModelCheckpoint(monitor="val_loss")
    return [early_stop, checkpoint]

configure_gradient_clipping(optimizer, gradient_clip_val=None, gradient_clip_algorithm=None)

Perform gradient clipping for the optimizer parameters. Called before optimizer_step().

Parameters

• optimizer (Optimizer) – Current optimizer being used.

• gradient_clip_val (Union[int, float, None]) – The value at which to clip gradients. By default, value passed in Trainer will be available here.

• gradient_clip_algorithm (Optional[str]) – The gradient clipping algorithm to use. By default, value passed in Trainer will be available here.

Example:

def configure_gradient_clipping(self, optimizer, gradient_clip_val, gradient_clip_algorithm):
    # Implement your own custom logic to clip gradients
    # You can call `self.clip_gradients` with your settings:
    self.clip_gradients(
        optimizer,
        gradient_clip_val=gradient_clip_val,
        gradient_clip_algorithm=gradient_clip_algorithm
    )

Return type

None

configure_optimizers()

Choose what optimizers and learning-rate schedulers to use in your optimization. Normally you’d need one. But in the case of GANs or similar you might have multiple. Optimization with multiple optimizers only works in the manual optimization mode.

Return type

Any

Returns

Any of these 6 options.

• Single optimizer.

• List or Tuple of optimizers.

• Two lists - The first list has multiple optimizers, and the second has multiple LR schedulers (or multiple lr_scheduler_config).

• Dictionary, with an "optimizer" key, and (optionally) a "lr_scheduler" key whose value is a single LR scheduler or lr_scheduler_config.

• None - Fit will run without any optimizer.

The lr_scheduler_config is a dictionary which contains the scheduler and its associated configuration. The default configuration is shown below.

lr_scheduler_config = {
    # REQUIRED: The scheduler instance
    "scheduler": lr_scheduler,
    # The unit of the scheduler's step size, could also be 'step'.
    # 'epoch' updates the scheduler on epoch end whereas 'step'
    # updates it after a optimizer update.
    "interval": "epoch",
    # How many epochs/steps should pass between calls to
    # `scheduler.step()`. 1 corresponds to updating the learning
    # rate after every epoch/step.
    "frequency": 1,
    # Metric to to monitor for schedulers like `ReduceLROnPlateau`
    "monitor": "val_loss",
    # If set to `True`, will enforce that the value specified 'monitor'
    # is available when the scheduler is updated, thus stopping
    # training if not found. If set to `False`, it will only produce a warning
    "strict": True,
    # If using the `LearningRateMonitor` callback to monitor the
    # learning rate progress, this keyword can be used to specify
    # a custom logged name
    "name": None,
}

When there are schedulers in which the .step() method is conditioned on a value, such as the torch.optim.lr_scheduler.ReduceLROnPlateau scheduler, Lightning requires that the lr_scheduler_config contains the keyword "monitor" set to the metric name that the scheduler should be conditioned on.

# The ReduceLROnPlateau scheduler requires a monitor
def configure_optimizers(self):
    optimizer = Adam(...)
    return {
        "optimizer": optimizer,
        "lr_scheduler": {
            "scheduler": ReduceLROnPlateau(optimizer, ...),
            "monitor": "metric_to_track",
            "frequency": "indicates how often the metric is updated"
            # If "monitor" references validation metrics, then "frequency" should be set to a
            # multiple of "trainer.check_val_every_n_epoch".
        },
    }


# In the case of two optimizers, only one using the ReduceLROnPlateau scheduler
def configure_optimizers(self):
    optimizer1 = Adam(...)
    optimizer2 = SGD(...)
    scheduler1 = ReduceLROnPlateau(optimizer1, ...)
    scheduler2 = LambdaLR(optimizer2, ...)
    return (
        {
            "optimizer": optimizer1,
            "lr_scheduler": {
                "scheduler": scheduler1,
                "monitor": "metric_to_track",
            },
        },
        {"optimizer": optimizer2, "lr_scheduler": scheduler2},
    )

Metrics can be made available to monitor by simply logging it using self.log('metric_to_track', metric_val) in your LightningModule.

Note

Some things to know:

• Lightning calls .backward() and .step() automatically in case of automatic optimization.

• If a learning rate scheduler is specified in configure_optimizers() with key "interval" (default “epoch”) in the scheduler configuration, Lightning will call the scheduler’s .step() method automatically in case of automatic optimization.

• If you use 16-bit precision (precision=16), Lightning will automatically handle the optimizer.

• If you use torch.optim.LBFGS, Lightning handles the closure function automatically for you.

• If you use multiple optimizers, you will have to switch to ‘manual optimization’ mode and step them yourself.

• If you need to control how often the optimizer steps, override the optimizer_step() hook.

forward(*args, **kwargs)

Same as torch.nn.Module.forward().

Parameters

• *args – Whatever you decide to pass into the forward method.

• **kwargs – Keyword arguments are also possible.

Return type

Any

Returns

Your model’s output

freeze()

Freeze all params for inference.

Example:

model = MyLightningModule(...)
model.freeze()

Return type

None

classmethod load_from_checkpoint(checkpoint_path, map_location=None, hparams_file=None, strict=True, **kwargs)

Primary way of loading a model from a checkpoint. When Lightning saves a checkpoint it stores the arguments passed to __init__ in the checkpoint under "hyper_parameters".

Any arguments specified through **kwargs will override args stored in "hyper_parameters".

Parameters

• checkpoint_path (Union[str, Path, IO]) – Path to checkpoint. This can also be a URL, or file-like object

• map_location (Union[device, str, int, Callable[[Union[device, str, int]], Union[device, str, int]], Dict[Union[device, str, int], Union[device, str, int]], None]) – If your checkpoint saved a GPU model and you now load on CPUs or a different number of GPUs, use this to map to the new setup. The behaviour is the same as in torch.load().

• hparams_file (Union[str, Path, None]) –

  Optional path to a .yaml or .csv file with hierarchical structure as in this example:

  drop_prob: 0.2
  dataloader:
      batch_size: 32
  

  You most likely won’t need this since Lightning will always save the hyperparameters to the checkpoint. However, if your checkpoint weights don’t have the hyperparameters saved, use this method to pass in a .yaml file with the hparams you’d like to use. These will be converted into a dict and passed into your LightningModule for use.

  If your model’s hparams argument is Namespace and .yaml file has hierarchical structure, you need to refactor your model to treat hparams as dict.

• strict (bool) – Whether to strictly enforce that the keys in checkpoint_path match the keys returned by this module’s state dict.

• **kwargs – Any extra keyword args needed to init the model. Can also be used to override saved hyperparameter values.

Return type

Self

Returns

LightningModule instance with loaded weights and hyperparameters (if available).

Note

load_from_checkpoint is a class method. You should use your LightningModule class to call it instead of the LightningModule instance.

Example:

# load weights without mapping ...
model = MyLightningModule.load_from_checkpoint('path/to/checkpoint.ckpt')

# or load weights mapping all weights from GPU 1 to GPU 0 ...
map_location = {'cuda:1':'cuda:0'}
model = MyLightningModule.load_from_checkpoint(
    'path/to/checkpoint.ckpt',
    map_location=map_location
)

# or load weights and hyperparameters from separate files.
model = MyLightningModule.load_from_checkpoint(
    'path/to/checkpoint.ckpt',
    hparams_file='/path/to/hparams_file.yaml'
)

# override some of the params with new values
model = MyLightningModule.load_from_checkpoint(
    PATH,
    num_layers=128,
    pretrained_ckpt_path=NEW_PATH,
)

# predict
pretrained_model.eval()
pretrained_model.freeze()
y_hat = pretrained_model(x)

log(name, value, prog_bar=False, logger=None, on_step=None, on_epoch=None, reduce_fx='mean', enable_graph=False, sync_dist=False, sync_dist_group=None, add_dataloader_idx=True, batch_size=None, metric_attribute=None, rank_zero_only=False)

Log a key, value pair.

Example:

self.log('train_loss', loss)

The default behavior per hook is documented here: Automatic Logging.

Parameters

• name (str) – key to log.

• value (Union[Metric, Tensor, int, float]) – value to log. Can be a float, Tensor, or a Metric.

• prog_bar (bool) – if True logs to the progress bar.

• logger (Optional[bool]) – if True logs to the logger.

• on_step (Optional[bool]) – if True logs at this step. The default value is determined by the hook. See Automatic Logging for details.

• on_epoch (Optional[bool]) – if True logs epoch accumulated metrics. The default value is determined by the hook. See Automatic Logging for details.

• reduce_fx (Union[str, Callable]) – reduction function over step values for end of epoch. torch.mean() by default.

• enable_graph (bool) – if True, will not auto detach the graph.

• sync_dist (bool) – if True, reduces the metric across devices. Use with care as this may lead to a significant communication overhead.

• sync_dist_group (Optional[Any]) – the DDP group to sync across.

• add_dataloader_idx (bool) – if True, appends the index of the current dataloader to the name (when using multiple dataloaders). If False, user needs to give unique names for each dataloader to not mix the values.

• batch_size (Optional[int]) – Current batch_size. This will be directly inferred from the loaded batch, but for some data structures you might need to explicitly provide it.

• metric_attribute (Optional[str]) – To restore the metric state, Lightning requires the reference of the torchmetrics.Metric in your model. This is found automatically if it is a model attribute.

• rank_zero_only (bool) – Whether the value will be logged only on rank 0. This will prevent synchronization which would produce a deadlock as not all processes would perform this log call.

Return type

None

log_dict(dictionary, prog_bar=False, logger=None, on_step=None, on_epoch=None, reduce_fx='mean', enable_graph=False, sync_dist=False, sync_dist_group=None, add_dataloader_idx=True, batch_size=None, rank_zero_only=False)

Log a dictionary of values at once.

Example:

values = {'loss': loss, 'acc': acc, ..., 'metric_n': metric_n}
self.log_dict(values)

Parameters

• dictionary (Mapping[str, Union[Metric, Tensor, int, float]]) – key value pairs. The values can be a float, Tensor, Metric, or MetricCollection.

• prog_bar (bool) – if True logs to the progress base.

• logger (Optional[bool]) – if True logs to the logger.

• on_step (Optional[bool]) – if True logs at this step. None auto-logs for training_step but not validation/test_step. The default value is determined by the hook. See Automatic Logging for details.

• on_epoch (Optional[bool]) – if True logs epoch accumulated metrics. None auto-logs for val/test step but not training_step. The default value is determined by the hook. See Automatic Logging for details.

• reduce_fx (Union[str, Callable]) – reduction function over step values for end of epoch. torch.mean() by default.

• enable_graph (bool) – if True, will not auto-detach the graph

• sync_dist (bool) – if True, reduces the metric across GPUs/TPUs. Use with care as this may lead to a significant communication overhead.

• sync_dist_group (Optional[Any]) – the ddp group to sync across.

• add_dataloader_idx (bool) – if True, appends the index of the current dataloader to the name (when using multiple). If False, user needs to give unique names for each dataloader to not mix values.

• batch_size (Optional[int]) – Current batch size. This will be directly inferred from the loaded batch, but some data structures might need to explicitly provide it.

• rank_zero_only (bool) – Whether the value will be logged only on rank 0. This will prevent synchronization which would produce a deadlock as not all processes would perform this log call.

Return type

None

lr_scheduler_step(scheduler, metric)

Override this method to adjust the default way the Trainer calls each scheduler. By default, Lightning calls step() and as shown in the example for each scheduler based on its interval.

Parameters

• scheduler (Union[_LRScheduler, ReduceLROnPlateau]) – Learning rate scheduler.

• metric (Optional[Any]) – Value of the monitor used for schedulers like ReduceLROnPlateau.

Examples:

# DEFAULT
def lr_scheduler_step(self, scheduler, metric):
    if metric is None:
        scheduler.step()
    else:
        scheduler.step(metric)

# Alternative way to update schedulers if it requires an epoch value
def lr_scheduler_step(self, scheduler, metric):
    scheduler.step(epoch=self.current_epoch)

Return type

None

lr_schedulers()

Returns the learning rate scheduler(s) that are being used during training. Useful for manual optimization.

Return type

Union[None, List[Union[LRScheduler, ReduceLROnPlateau]], LRScheduler, ReduceLROnPlateau]

Returns

A single scheduler, or a list of schedulers in case multiple ones are present, or None if no schedulers were returned in configure_optimizers().

manual_backward(loss, *args, **kwargs)

Call this directly from your training_step() when doing optimizations manually. By using this, Lightning can ensure that all the proper scaling gets applied when using mixed precision.

See manual optimization for more examples.

Example:

def training_step(...):
    opt = self.optimizers()
    loss = ...
    opt.zero_grad()
    # automatically applies scaling, etc...
    self.manual_backward(loss)
    opt.step()

Parameters

• loss (Tensor) – The tensor on which to compute gradients. Must have a graph attached.

• *args – Additional positional arguments to be forwarded to backward()

• **kwargs – Additional keyword arguments to be forwarded to backward()

Return type

None

optimizer_step(epoch, batch_idx, optimizer, optimizer_closure=None)

Override this method to adjust the default way the Trainer calls the optimizer.

By default, Lightning calls step() and zero_grad() as shown in the example. This method (and zero_grad()) won’t be called during the accumulation phase when Trainer(accumulate_grad_batches != 1). Overriding this hook has no benefit with manual optimization.

Parameters

• epoch (int) – Current epoch

• batch_idx (int) – Index of current batch

• optimizer (Union[Optimizer, LightningOptimizer]) – A PyTorch optimizer

• optimizer_closure (Optional[Callable[[], Any]]) – The optimizer closure. This closure must be executed as it includes the calls to training_step(), optimizer.zero_grad(), and backward().

Examples:

# DEFAULT
def optimizer_step(self, epoch, batch_idx, optimizer, optimizer_closure):
    optimizer.step(closure=optimizer_closure)

# Learning rate warm-up
def optimizer_step(self, epoch, batch_idx, optimizer, optimizer_closure):
    # update params
    optimizer.step(closure=optimizer_closure)

    # manually warm up lr without a scheduler
    if self.trainer.global_step < 500:
        lr_scale = min(1.0, float(self.trainer.global_step + 1) / 500.0)
        for pg in optimizer.param_groups:
            pg["lr"] = lr_scale * self.learning_rate

Return type

None

optimizer_zero_grad(epoch, batch_idx, optimizer)

Override this method to change the default behaviour of optimizer.zero_grad().

Parameters

• epoch (int) – Current epoch

• batch_idx (int) – Index of current batch

• optimizer (Optimizer) – A PyTorch optimizer

Examples:

# DEFAULT
def optimizer_zero_grad(self, epoch, batch_idx, optimizer):
    optimizer.zero_grad()

# Set gradients to `None` instead of zero to improve performance (not required on `torch>=2.0.0`).
def optimizer_zero_grad(self, epoch, batch_idx, optimizer):
    optimizer.zero_grad(set_to_none=True)

See torch.optim.Optimizer.zero_grad() for the explanation of the above example.

Return type

None

optimizers(use_pl_optimizer: Literal[True] = True) → Union[LightningOptimizer, List[LightningOptimizer]]

optimizers(use_pl_optimizer: Literal[False]) → Union[Optimizer, List[Optimizer]]

optimizers(use_pl_optimizer: bool) → Union[Optimizer, LightningOptimizer, _FabricOptimizer, List[Optimizer], List[LightningOptimizer], List[_FabricOptimizer]]

Returns the optimizer(s) that are being used during training. Useful for manual optimization.

Parameters

use_pl_optimizer (bool) – If True, will wrap the optimizer(s) in a LightningOptimizer for automatic handling of precision and profiling.

Return type

Union[Optimizer, LightningOptimizer, _FabricOptimizer, List[Optimizer], List[LightningOptimizer], List[_FabricOptimizer]]

Returns

A single optimizer, or a list of optimizers in case multiple ones are present.

predict_step(batch, batch_idx, dataloader_idx=0)

Step function called during predict(). By default, it calls forward(). Override to add any processing logic.

The predict_step() is used to scale inference on multi-devices.

To prevent an OOM error, it is possible to use BasePredictionWriter callback to write the predictions to disk or database after each batch or on epoch end.

The BasePredictionWriter should be used while using a spawn based accelerator. This happens for Trainer(strategy="ddp_spawn") or training on 8 TPU cores with Trainer(accelerator="tpu", devices=8) as predictions won’t be returned.

Example

class MyModel(LightningModule):

    def predict_step(self, batch, batch_idx, dataloader_idx=0):
        return self(batch)

dm = ...
model = MyModel()
trainer = Trainer(accelerator="gpu", devices=2)
predictions = trainer.predict(model, dm)

Parameters

• batch (Any) – Current batch.

• batch_idx (int) – Index of current batch.

• dataloader_idx (int) – Index of the current dataloader.

Return type

Any

Returns

Predicted output

print(*args, **kwargs)

Prints only from process 0. Use this in any distributed mode to log only once.

Parameters

• *args – The thing to print. The same as for Python’s built-in print function.

• **kwargs – The same as for Python’s built-in print function.

Example:

def forward(self, x):
    self.print(x, 'in forward')

Return type

None

test_step(*args, **kwargs)

Operates on a single batch of data from the test set. In this step you’d normally generate examples or calculate anything of interest such as accuracy.

Parameters

• batch – The output of your DataLoader.

• batch_idx – The index of this batch.

• dataloader_id – The index of the dataloader that produced this batch. (only if multiple test dataloaders used).

Return type

Union[Tensor, Dict[str, Any], None]

Returns

Any of.

• Any object or value

• None - Testing will skip to the next batch

# if you have one test dataloader:
def test_step(self, batch, batch_idx):
    ...


# if you have multiple test dataloaders:
def test_step(self, batch, batch_idx, dataloader_idx=0):
    ...

Examples:

# CASE 1: A single test dataset
def test_step(self, batch, batch_idx):
    x, y = batch

    # implement your own
    out = self(x)
    loss = self.loss(out, y)

    # log 6 example images
    # or generated text... or whatever
    sample_imgs = x[:6]
    grid = torchvision.utils.make_grid(sample_imgs)
    self.logger.experiment.add_image('example_images', grid, 0)

    # calculate acc
    labels_hat = torch.argmax(out, dim=1)
    test_acc = torch.sum(y == labels_hat).item() / (len(y) * 1.0)

    # log the outputs!
    self.log_dict({'test_loss': loss, 'test_acc': test_acc})

If you pass in multiple test dataloaders, test_step() will have an additional argument. We recommend setting the default value of 0 so that you can quickly switch between single and multiple dataloaders.

# CASE 2: multiple test dataloaders
def test_step(self, batch, batch_idx, dataloader_idx=0):
    # dataloader_idx tells you which dataset this is.
    ...

Note

If you don’t need to test you don’t need to implement this method.

Note

When the test_step() is called, the model has been put in eval mode and PyTorch gradients have been disabled. At the end of the test epoch, the model goes back to training mode and gradients are enabled.

to_onnx(file_path, input_sample=None, **kwargs)

Saves the model in ONNX format.

Parameters

• file_path (Union[str, Path]) – The path of the file the onnx model should be saved to.

• input_sample (Optional[Any]) – An input for tracing. Default: None (Use self.example_input_array)

• **kwargs – Will be passed to torch.onnx.export function.

Example:

class SimpleModel(LightningModule):
    def __init__(self):
        super().__init__()
        self.l1 = torch.nn.Linear(in_features=64, out_features=4)

    def forward(self, x):
        return torch.relu(self.l1(x.view(x.size(0), -1)

model = SimpleModel()
input_sample = torch.randn(1, 64)
model.to_onnx("export.onnx", input_sample, export_params=True)

Return type

None

to_torchscript(file_path=None, method='script', example_inputs=None, **kwargs)

By default compiles the whole model to a ScriptModule. If you want to use tracing, please provided the argument method='trace' and make sure that either the example_inputs argument is provided, or the model has example_input_array set. If you would like to customize the modules that are scripted you should override this method. In case you want to return multiple modules, we recommend using a dictionary.

Parameters

• file_path (Union[str, Path, None]) – Path where to save the torchscript. Default: None (no file saved).

• method (Optional[str]) – Whether to use TorchScript’s script or trace method. Default: ‘script’

• example_inputs (Optional[Any]) – An input to be used to do tracing when method is set to ‘trace’. Default: None (uses example_input_array)

• **kwargs – Additional arguments that will be passed to the torch.jit.script() or torch.jit.trace() function.

Note

• Requires the implementation of the forward() method.

• The exported script will be set to evaluation mode.

• It is recommended that you install the latest supported version of PyTorch to use this feature without limitations. See also the torch.jit documentation for supported features.

Example:

class SimpleModel(LightningModule):
    def __init__(self):
        super().__init__()
        self.l1 = torch.nn.Linear(in_features=64, out_features=4)

    def forward(self, x):
        return torch.relu(self.l1(x.view(x.size(0), -1)))

model = SimpleModel()
model.to_torchscript(file_path="model.pt")

torch.jit.save(model.to_torchscript(
    file_path="model_trace.pt", method='trace', example_inputs=torch.randn(1, 64))
)

Return type

Union[ScriptModule, Dict[str, ScriptModule]]

Returns

This LightningModule as a torchscript, regardless of whether file_path is defined or not.

toggle_optimizer(optimizer)

Makes sure only the gradients of the current optimizer’s parameters are calculated in the training step to prevent dangling gradients in multiple-optimizer setup.

It works with untoggle_optimizer() to make sure param_requires_grad_state is properly reset.

Parameters

optimizer (Union[Optimizer, LightningOptimizer]) – The optimizer to toggle.

Return type

None

training_step(*args, **kwargs)

Here you compute and return the training loss and some additional metrics for e.g. the progress bar or logger.

Parameters

• batch (Tensor | (Tensor, …) | [Tensor, …]) – The output of your DataLoader. A tensor, tuple or list.

• batch_idx (int) – Integer displaying index of this batch

Return type

Union[Tensor, Dict[str, Any]]

Returns

Any of.

• Tensor - The loss tensor

• dict - A dictionary. Can include any keys, but must include the key 'loss'

• None - Training will skip to the next batch. This is only for automatic optimization.

  This is not supported for multi-GPU, TPU, IPU, or DeepSpeed.

In this step you’d normally do the forward pass and calculate the loss for a batch. You can also do fancier things like multiple forward passes or something model specific.

Example:

def training_step(self, batch, batch_idx):
    x, y, z = batch
    out = self.encoder(x)
    loss = self.loss(out, x)
    return loss

To use multiple optimizers, you can switch to ‘manual optimization’ and control their stepping:

def __init__(self):
    super().__init__()
    self.automatic_optimization = False


# Multiple optimizers (e.g.: GANs)
def training_step(self, batch, batch_idx):
    opt1, opt2 = self.optimizers()

    # do training_step with encoder
    ...
    opt1.step()
    # do training_step with decoder
    ...
    opt2.step()

Note

When accumulate_grad_batches > 1, the loss returned here will be automatically normalized by accumulate_grad_batches internally.

unfreeze()

Unfreeze all parameters for training.

model = MyLightningModule(...)
model.unfreeze()

Return type

None

untoggle_optimizer(optimizer)

Resets the state of required gradients that were toggled with toggle_optimizer().

Parameters

optimizer (Union[Optimizer, LightningOptimizer]) – The optimizer to untoggle.

Return type

None

validation_step(*args, **kwargs)

Operates on a single batch of data from the validation set. In this step you’d might generate examples or calculate anything of interest like accuracy.

Parameters

• batch – The output of your DataLoader.

• batch_idx – The index of this batch.

• dataloader_idx – The index of the dataloader that produced this batch. (only if multiple val dataloaders used)

Return type

Union[Tensor, Dict[str, Any], None]

Returns

• Any object or value

• None - Validation will skip to the next batch

# if you have one val dataloader:
def validation_step(self, batch, batch_idx):
    ...


# if you have multiple val dataloaders:
def validation_step(self, batch, batch_idx, dataloader_idx=0):
    ...

Examples:

# CASE 1: A single validation dataset
def validation_step(self, batch, batch_idx):
    x, y = batch

    # implement your own
    out = self(x)
    loss = self.loss(out, y)

    # log 6 example images
    # or generated text... or whatever
    sample_imgs = x[:6]
    grid = torchvision.utils.make_grid(sample_imgs)
    self.logger.experiment.add_image('example_images', grid, 0)

    # calculate acc
    labels_hat = torch.argmax(out, dim=1)
    val_acc = torch.sum(y == labels_hat).item() / (len(y) * 1.0)

    # log the outputs!
    self.log_dict({'val_loss': loss, 'val_acc': val_acc})

If you pass in multiple val dataloaders, validation_step() will have an additional argument. We recommend setting the default value of 0 so that you can quickly switch between single and multiple dataloaders.

# CASE 2: multiple validation dataloaders
def validation_step(self, batch, batch_idx, dataloader_idx=0):
    # dataloader_idx tells you which dataset this is.
    ...

Note

If you don’t need to validate you don’t need to implement this method.

Note

When the validation_step() is called, the model has been put in eval mode and PyTorch gradients have been disabled. At the end of validation, the model goes back to training mode and gradients are enabled.

property automatic_optimization: bool

If set to False you are responsible for calling .backward(), .step(), .zero_grad().

Return type

bool

property current_epoch: int

The current epoch in the Trainer, or 0 if not attached.

Return type

int

property example_input_array: Optional[Union[Tensor, Tuple, Dict]]

The example input array is a specification of what the module can consume in the forward() method. The return type is interpreted as follows:

• Single tensor: It is assumed the model takes a single argument, i.e., model.forward(model.example_input_array)

• Tuple: The input array should be interpreted as a sequence of positional arguments, i.e., model.forward(*model.example_input_array)

• Dict: The input array represents named keyword arguments, i.e., model.forward(**model.example_input_array)

Return type

Union[Tensor, Tuple, Dict, None]

property global_rank: int

The index of the current process across all nodes and devices.

Return type

int

property global_step: int

Total training batches seen across all epochs.

If no Trainer is attached, this propery is 0.

Return type

int

property local_rank: int

The index of the current process within a single node.

Return type

int

property logger: Optional[Union[Logger, Logger]]

Reference to the logger object in the Trainer.

Return type

Union[Logger, Logger, None]

property loggers: Union[List[Logger], List[Logger]]

Reference to the list of loggers in the Trainer.

Return type

Union[List[Logger], List[Logger]]

property on_gpu: bool

Returns True if this model is currently located on a GPU.

Useful to set flags around the LightningModule for different CPU vs GPU behavior.

Return type

bool