LightningModule¶
A LightningModule
organizes your PyTorch code into 5 sections
Computations (init).
Train loop (training_step)
Validation loop (validation_step)
Test loop (test_step)
Optimizers (configure_optimizers)
Notice a few things.
It’s the SAME code.
The PyTorch code IS NOT abstracted - just organized.
All the other code that’s not in the
LightningModule
has been automated for you by the trainer.
net = Net() trainer = Trainer() trainer.fit(net)
There are no .cuda() or .to() calls… Lightning does these for you.
# don't do in lightning x = torch.Tensor(2, 3) x = x.cuda() x = x.to(device) # do this instead x = x # leave it alone! # or to init a new tensor new_x = torch.Tensor(2, 3) new_x = new_x.type_as(x)
Lightning by default handles the distributed sampler for you.
# Don't do in Lightning... data = MNIST(...) sampler = DistributedSampler(data) DataLoader(data, sampler=sampler) # do this instead data = MNIST(...) DataLoader(data)
A
LightningModule
is atorch.nn.Module
but with added functionality. Use it as such!
net = Net.load_from_checkpoint(PATH) net.freeze() out = net(x)
Thus, to use Lightning, you just need to organize your code which takes about 30 minutes, (and let’s be real, you probably should do anyhow).
Minimal Example¶
Here are the only required methods.
>>> import pytorch_lightning as pl
>>> class LitModel(pl.LightningModule):
...
... def __init__(self):
... super().__init__()
... self.l1 = nn.Linear(28 * 28, 10)
...
... def forward(self, x):
... return torch.relu(self.l1(x.view(x.size(0), -1)))
...
... def training_step(self, batch, batch_idx):
... x, y = batch
... y_hat = self(x)
... loss = F.cross_entropy(y_hat, y)
... return loss
...
... def configure_optimizers(self):
... return torch.optim.Adam(self.parameters(), lr=0.02)
Which you can train by doing:
train_loader = DataLoader(MNIST(os.getcwd(), download=True, transform=transforms.ToTensor()))
trainer = pl.Trainer()
model = LitModel()
trainer.fit(model, train_loader)
The LightningModule has many convenience methods, but the core ones you need to know about are:
Name |
Description |
---|---|
init |
Define computations here |
forward |
Use for inference only (separate from training_step) |
training_step |
the full training loop |
validation_step |
the full validation loop |
test_step |
the full test loop |
configure_optimizers |
define optimizers and LR schedulers |
Training¶
Training loop¶
To add a training loop use the training_step method
class LitClassifier(pl.LightningModule):
def __init__(self, model):
super().__init__()
self.model = model
def training_step(self, batch, batch_idx):
x, y = batch
y_hat = self.model(x)
loss = F.cross_entropy(y_hat, y)
return loss
Under the hood, Lightning does the following (pseudocode):
# put model in train mode
model.train()
torch.set_grad_enabled(True)
losses = []
for batch in train_dataloader:
# forward
loss = training_step(batch)
losses.append(loss.detach())
# clear gradients
optimizer.zero_grad()
# backward
loss.backward()
# update parameters
optimizer.step()
Training epoch-level metrics¶
If you want to calculate epoch-level metrics and log them, use the .log method
def training_step(self, batch, batch_idx):
x, y = batch
y_hat = self.model(x)
loss = F.cross_entropy(y_hat, y)
# logs metrics for each training_step,
# and the average across the epoch, to the progress bar and logger
self.log('train_loss', loss, on_step=True, on_epoch=True, prog_bar=True, logger=True)
return loss
The .log object automatically reduces the requested metrics across the full epoch. Here’s the pseudocode of what it does under the hood:
outs = []
for batch in train_dataloader:
# forward
out = training_step(val_batch)
outs.append(out)
# clear gradients
optimizer.zero_grad()
# backward
loss.backward()
# update parameters
optimizer.step()
epoch_metric = torch.mean(torch.stack([x['train_loss'] for x in outs]))
Train epoch-level operations¶
If you need to do something with all the outputs of each training_step, override training_epoch_end yourself.
def training_step(self, batch, batch_idx):
x, y = batch
y_hat = self.model(x)
loss = F.cross_entropy(y_hat, y)
preds = ...
return {'loss': loss, 'other_stuff': preds}
def training_epoch_end(self, training_step_outputs):
for pred in training_step_outputs:
# do something
The matching pseudocode is:
outs = []
for batch in train_dataloader:
# forward
out = training_step(val_batch)
outs.append(out)
# clear gradients
optimizer.zero_grad()
# backward
loss.backward()
# update parameters
optimizer.step()
training_epoch_end(outs)
Training with DataParallel¶
When training using a accelerator that splits data from each batch across GPUs, sometimes you might need to aggregate them on the master GPU for processing (dp, or ddp2).
In this case, implement the training_step_end method
def training_step(self, batch, batch_idx):
x, y = batch
y_hat = self.model(x)
loss = F.cross_entropy(y_hat, y)
pred = ...
return {'loss': loss, 'pred': pred}
def training_step_end(self, batch_parts):
# predictions from each GPU
predictions = batch_parts['pred']
# losses from each GPU
losses = batch_parts['loss']
gpu_0_prediction = predictions[0]
gpu_1_prediction = predictions[1]
# do something with both outputs
return (losses[0] + losses[1]) / 2
def training_epoch_end(self, training_step_outputs):
for out in training_step_outputs:
# do something with preds
The full pseudocode that lighting does under the hood is:
outs = []
for train_batch in train_dataloader:
batches = split_batch(train_batch)
dp_outs = []
for sub_batch in batches:
# 1
dp_out = training_step(sub_batch)
dp_outs.append(dp_out)
# 2
out = training_step_end(dp_outs)
outs.append(out)
# do something with the outputs for all batches
# 3
training_epoch_end(outs)
Validation loop¶
To add a validation loop, override the validation_step method of the LightningModule
:
class LitModel(pl.LightningModule):
def validation_step(self, batch, batch_idx):
x, y = batch
y_hat = self.model(x)
loss = F.cross_entropy(y_hat, y)
self.log('val_loss', loss)
Under the hood, Lightning does the following:
# ...
for batch in train_dataloader:
loss = model.training_step()
loss.backward()
# ...
if validate_at_some_point:
# disable grads + batchnorm + dropout
torch.set_grad_enabled(False)
model.eval()
# ----------------- VAL LOOP ---------------
for val_batch in model.val_dataloader:
val_out = model.validation_step(val_batch)
# ----------------- VAL LOOP ---------------
# enable grads + batchnorm + dropout
torch.set_grad_enabled(True)
model.train()
Validation epoch-level metrics¶
If you need to do something with all the outputs of each validation_step, override validation_epoch_end.
def validation_step(self, batch, batch_idx):
x, y = batch
y_hat = self.model(x)
loss = F.cross_entropy(y_hat, y)
pred = ...
return pred
def validation_epoch_end(self, validation_step_outputs):
for pred in validation_step_outputs:
# do something with a pred
Validating with DataParallel¶
When training using a accelerator that splits data from each batch across GPUs, sometimes you might need to aggregate them on the master GPU for processing (dp, or ddp2).
In this case, implement the validation_step_end method
def validation_step(self, batch, batch_idx):
x, y = batch
y_hat = self.model(x)
loss = F.cross_entropy(y_hat, y)
pred = ...
return {'loss': loss, 'pred': pred}
def validation_step_end(self, batch_parts):
# predictions from each GPU
predictions = batch_parts['pred']
# losses from each GPU
losses = batch_parts['loss']
gpu_0_prediction = predictions[0]
gpu_1_prediction = predictions[1]
# do something with both outputs
return (losses[0] + losses[1]) / 2
def validation_epoch_end(self, validation_step_outputs):
for out in validation_step_outputs:
# do something with preds
The full pseudocode that lighting does under the hood is:
outs = []
for batch in dataloader:
batches = split_batch(batch)
dp_outs = []
for sub_batch in batches:
# 1
dp_out = validation_step(sub_batch)
dp_outs.append(dp_out)
# 2
out = validation_step_end(dp_outs)
outs.append(out)
# do something with the outputs for all batches
# 3
validation_epoch_end(outs)
Test loop¶
The process for adding a test loop is the same as the process for adding a validation loop. Please refer to the section above for details.
The only difference is that the test loop is only called when .test() is used:
model = Model()
trainer = Trainer()
trainer.fit()
# automatically loads the best weights for you
trainer.test(model)
There are two ways to call test():
# call after training
trainer = Trainer()
trainer.fit(model)
# automatically auto-loads the best weights
trainer.test(dataloaders=test_dataloader)
# or call with pretrained model
model = MyLightningModule.load_from_checkpoint(PATH)
trainer = Trainer()
trainer.test(model, dataloaders=test_dataloader)
Inference¶
For research, LightningModules are best structured as systems.
import pytorch_lightning as pl
import torch
from torch import nn
class Autoencoder(pl.LightningModule):
def __init__(self, latent_dim=2):
super().__init__()
self.encoder = nn.Sequential(nn.Linear(28 * 28, 256), nn.ReLU(), nn.Linear(256, latent_dim))
self.decoder = nn.Sequential(nn.Linear(latent_dim, 256), nn.ReLU(), nn.Linear(256, 28 * 28))
def training_step(self, batch, batch_idx):
x, _ = batch
# encode
x = x.view(x.size(0), -1)
z = self.encoder(x)
# decode
recons = self.decoder(z)
# reconstruction
reconstruction_loss = nn.functional.mse_loss(recons, x)
return reconstruction_loss
def validation_step(self, batch, batch_idx):
x, _ = batch
x = x.view(x.size(0), -1)
z = self.encoder(x)
recons = self.decoder(z)
reconstruction_loss = nn.functional.mse_loss(recons, x)
self.log('val_reconstruction', reconstruction_loss)
def predict_step(self, batch, batch_idx, dataloader_idx):
x, _ = batch
# encode
# for predictions, we could return the embedding or the reconstruction or both based on our need.
x = x.view(x.size(0), -1)
return self.encoder(x)
def configure_optimizers(self):
return torch.optim.Adam(self.parameters(), lr=0.0002)
Which can be trained like this:
autoencoder = Autoencoder()
trainer = pl.Trainer(gpus=1)
trainer.fit(autoencoder, train_dataloader, val_dataloader)
This simple model generates examples that look like this (the encoders and decoders are too weak)
![https://pl-bolts-doc-images.s3.us-east-2.amazonaws.com/pl_docs/ae_docs.png](https://pl-bolts-doc-images.s3.us-east-2.amazonaws.com/pl_docs/ae_docs.png)
The methods above are part of the lightning interface:
training_step
validation_step
test_step
predict_step
configure_optimizers
Note that in this case, the train loop and val loop are exactly the same. We can of course reuse this code.
class Autoencoder(pl.LightningModule):
def __init__(self, latent_dim=2):
super().__init__()
self.encoder = nn.Sequential(nn.Linear(28 * 28, 256), nn.ReLU(), nn.Linear(256, latent_dim))
self.decoder = nn.Sequential(nn.Linear(latent_dim, 256), nn.ReLU(), nn.Linear(256, 28 * 28))
def training_step(self, batch, batch_idx):
loss = self.shared_step(batch)
return loss
def validation_step(self, batch, batch_idx):
loss = self.shared_step(batch)
self.log('val_loss', loss)
def shared_step(self, batch):
x, _ = batch
# encode
x = x.view(x.size(0), -1)
z = self.encoder(x)
# decode
recons = self.decoder(z)
# loss
return nn.functional.mse_loss(recons, x)
def configure_optimizers(self):
return torch.optim.Adam(self.parameters(), lr=0.0002)
We create a new method called shared_step that all loops can use. This method name is arbitrary and NOT reserved.
Inference in research¶
In the case where we want to perform inference with the system we can add a forward method to the LightningModule.
Note
When using forward, you are responsible to call eval()
and use the no_grad()
context manager.
class Autoencoder(pl.LightningModule):
def forward(self, x):
return self.decoder(x)
model = Autoencoder()
model.eval()
with torch.no_grad():
reconstruction = model(embedding)
The advantage of adding a forward is that in complex systems, you can do a much more involved inference procedure, such as text generation:
class Seq2Seq(pl.LightningModule):
def forward(self, x):
embeddings = self(x)
hidden_states = self.encoder(embeddings)
for h in hidden_states:
# decode
...
return decoded
In the case where you want to scale your inference, you should be using
predict_step()
.
class Autoencoder(pl.LightningModule):
def forward(self, x):
return self.decoder(x)
def predict_step(self, batch, batch_idx, dataloader_idx = None)
# this calls forward
return self(batch)
data_module = ...
model = Autoencoder()
trainer = Trainer(gpus=2)
trainer.predict(model, data_module)
Inference in production¶
For cases like production, you might want to iterate different models inside a LightningModule.
import pytorch_lightning as pl
from pytorch_lightning.metrics import functional as FM
class ClassificationTask(pl.LightningModule):
def __init__(self, model):
super().__init__()
self.model = model
def training_step(self, batch, batch_idx):
x, y = batch
y_hat = self.model(x)
loss = F.cross_entropy(y_hat, y)
return loss
def validation_step(self, batch, batch_idx):
loss, acc = self._shared_eval_step(batch, batch_idx)
metrics = {'val_acc': acc, 'val_loss': loss}
self.log_dict(metrics)
return metrics
def test_step(self, batch, batch_idx):
loss, acc = self._shared_eval_step(batch, batch_idx)
metrics = {'test_acc': acc, 'test_loss': loss}
self.log_dict(metrics)
return metrics
def _shared_eval_step(self, batch, batch_idx):
x, y = batch
y_hat = self.model(x)
loss = F.cross_entropy(y_hat, y)
acc = FM.accuracy(y_hat, y)
return loss, acc
def predict_step(self, batch, batch_idx, dataloader_idx):
x, y = batch
y_hat = self.model(x)
def configure_optimizers(self):
return torch.optim.Adam(self.model.parameters(), lr=0.02)
Then pass in any arbitrary model to be fit with this task
for model in [resnet50(), vgg16(), BidirectionalRNN()]:
task = ClassificationTask(model)
trainer = Trainer(gpus=2)
trainer.fit(task, train_dataloader, val_dataloader)
Tasks can be arbitrarily complex such as implementing GAN training, self-supervised or even RL.
class GANTask(pl.LightningModule):
def __init__(self, generator, discriminator):
super().__init__()
self.generator = generator
self.discriminator = discriminator
...
When used like this, the model can be separated from the Task and thus used in production without needing to keep it in a LightningModule.
You can export to onnx.
Or trace using Jit.
or run in the python runtime.
task = ClassificationTask(model)
trainer = Trainer(gpus=2)
trainer.fit(task, train_dataloader, val_dataloader)
# use model after training or load weights and drop into the production system
model.eval()
y_hat = model(x)
LightningModule API¶
Methods¶
configure_callbacks¶
- LightningModule.configure_callbacks()[source]
Configure model-specific callbacks. When the model gets attached, e.g., when
.fit()
or.test()
gets called, the list returned here will be merged with the list of callbacks passed to the Trainer’scallbacks
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 sureModelCheckpoint
callbacks run last.- Returns
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]
Note
Certain callback methods like
on_init_start()
will never be invoked on the new callbacks returned here.
configure_optimizers¶
- LightningModule.configure_optimizers()[source]
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.
- 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_dict
).Dictionary, with an
"optimizer"
key, and (optionally) a"lr_scheduler"
key whose value is a single LR scheduler orlr_dict
.Tuple of dictionaries as described above, with an optional
"frequency"
key.None - Fit will run without any optimizer.
The
lr_dict
is a dictionary which contains the scheduler and its associated configuration. The default configuration is shown below.lr_dict = { # 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 thetorch.optim.lr_scheduler.ReduceLROnPlateau
scheduler, Lightning requires that thelr_dict
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', } } # 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 yourLightningModule
.Note
The
frequency
value specified in a dict along with theoptimizer
key is an int corresponding to the number of sequential batches optimized with the specific optimizer. It should be given to none or to all of the optimizers. There is a difference between passing multiple optimizers in a list, and passing multiple optimizers in dictionaries with a frequency of 1:In the former case, all optimizers will operate on the given batch in each optimization step.
In the latter, only one optimizer will operate on the given batch at every step.
This is different from the
frequency
value specified in thelr_dict
mentioned above.def configure_optimizers(self): optimizer_one = torch.optim.SGD(self.model.parameters(), lr=0.01) optimizer_two = torch.optim.SGD(self.model.parameters(), lr=0.01) return [ {'optimizer': optimizer_one, 'frequency': 5}, {'optimizer': optimizer_two, 'frequency': 10}, ]
In this example, the first optimizer will be used for the first 5 steps, the second optimizer for the next 10 steps and that cycle will continue. If an LR scheduler is specified for an optimizer using the
lr_scheduler
key in the above dict, the scheduler will only be updated when its optimizer is being used.Examples:
# most cases. no learning rate scheduler def configure_optimizers(self): return Adam(self.parameters(), lr=1e-3) # multiple optimizer case (e.g.: GAN) def configure_optimizers(self): gen_opt = Adam(self.model_gen.parameters(), lr=0.01) dis_opt = Adam(self.model_dis.parameters(), lr=0.02) return gen_opt, dis_opt # example with learning rate schedulers def configure_optimizers(self): gen_opt = Adam(self.model_gen.parameters(), lr=0.01) dis_opt = Adam(self.model_dis.parameters(), lr=0.02) dis_sch = CosineAnnealing(dis_opt, T_max=10) return [gen_opt, dis_opt], [dis_sch] # example with step-based learning rate schedulers # each optimizer has its own scheduler def configure_optimizers(self): gen_opt = Adam(self.model_gen.parameters(), lr=0.01) dis_opt = Adam(self.model_dis.parameters(), lr=0.02) gen_sch = { 'scheduler': ExponentialLR(gen_opt, 0.99), 'interval': 'step' # called after each training step } dis_sch = CosineAnnealing(dis_opt, T_max=10) # called every epoch return [gen_opt, dis_opt], [gen_sch, dis_sch] # example with optimizer frequencies # see training procedure in `Improved Training of Wasserstein GANs`, Algorithm 1 # https://arxiv.org/abs/1704.00028 def configure_optimizers(self): gen_opt = Adam(self.model_gen.parameters(), lr=0.01) dis_opt = Adam(self.model_dis.parameters(), lr=0.02) n_critic = 5 return ( {'optimizer': dis_opt, 'frequency': n_critic}, {'optimizer': gen_opt, 'frequency': 1} )
Note
Some things to know:
Lightning calls
.backward()
and.step()
on each optimizer and learning rate scheduler as needed.If you use 16-bit precision (
precision=16
), Lightning will automatically handle the optimizers.If you use multiple optimizers,
training_step()
will have an additionaloptimizer_idx
parameter.If you use
torch.optim.LBFGS
, Lightning handles the closure function automatically for you.If you use multiple optimizers, gradients will be calculated only for the parameters of current optimizer at each training step.
If you need to control how often those optimizers step or override the default
.step()
schedule, override theoptimizer_step()
hook.
forward¶
- LightningModule.forward(*args, **kwargs)[source]
Same as
torch.nn.Module.forward()
.
freeze¶
log¶
- LightningModule.log(name, value, prog_bar=False, logger=True, on_step=None, on_epoch=None, reduce_fx='default', tbptt_reduce_fx=None, tbptt_pad_token=None, enable_graph=False, sync_dist=False, sync_dist_op=None, sync_dist_group=None, add_dataloader_idx=True, batch_size=None, metric_attribute=None, rank_zero_only=None)[source]
Log a key, value pair.
Example:
self.log('train_loss', loss)
The default behavior per hook is as follows:
*
also applies to the test loop¶LightningModule Hook
on_step
on_epoch
prog_bar
logger
training_step
T
F
F
T
training_step_end
T
F
F
T
training_epoch_end
F
T
F
T
validation_step*
F
T
F
T
validation_step_end*
F
T
F
T
validation_epoch_end*
F
T
F
T
- Parameters
name¶ – key to log
value¶ – value to log. Can be a
float
,Tensor
,Metric
, or a dictionary of the former.prog_bar¶ – if True logs to the progress bar
logger¶ – if True logs to the logger
on_step¶ – if True logs at this step. None auto-logs at the training_step but not validation/test_step
on_epoch¶ – if True logs epoch accumulated metrics. None auto-logs at the val/test step but not training_step
reduce_fx¶ – reduction function over step values for end of epoch.
torch.mean()
by default.enable_graph¶ – if True, will not auto detach the graph
sync_dist¶ – if True, reduces the metric across GPUs/TPUs
sync_dist_group¶ – the ddp group to sync across
add_dataloader_idx¶ – 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¶ – Current batch_size. This will be directly inferred from the loaded batch, but some data structures might need to explicitly provide it.
metric_attribute¶ – 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¶ – 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.
log_dict¶
- LightningModule.log_dict(dictionary, prog_bar=False, logger=True, on_step=None, on_epoch=None, reduce_fx='default', tbptt_reduce_fx=None, tbptt_pad_token=None, enable_graph=False, sync_dist=False, sync_dist_op=None, sync_dist_group=None, add_dataloader_idx=True)[source]
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
,Number
,Mapping
[str
,Union
[Metric
,Tensor
,Number
]]]]) – key value pairs. The values can be afloat
,Tensor
,Metric
, or a dictionary of the former.on_step¶ (
Optional
[bool
]) – if True logs at this step. None auto-logs for training_step but not validation/test_stepon_epoch¶ (
Optional
[bool
]) – if True logs epoch accumulated metrics. None auto-logs for val/test step but not training_stepreduce_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 graphsync_dist¶ (
bool
) – if True, reduces the metric across GPUs/TPUssync_dist_group¶ (
Optional
[Any
]) – the ddp group sync acrossadd_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
- Return type
manual_backward¶
- LightningModule.manual_backward(loss, *args, **kwargs)[source]
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
print¶
- LightningModule.print(*args, **kwargs)[source]
Prints only from process 0. Use this in any distributed mode to log only once.
- Parameters
- Return type
Example:
def forward(self, x): self.print(x, 'in forward')
predict_step¶
- LightningModule.predict_step(batch, batch_idx, dataloader_idx=None)[source]
Step function called during
predict()
. By default, it callsforward()
. 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 forTrainer(accelerator="ddp_spawn")
or training on 8 TPU cores withTrainer(tpu_cores=8)
as predictions won’t be returned.Example
class MyModel(LightningModule): def predicts_step(self, batch, batch_idx, dataloader_idx): return self(batch) dm = ... model = MyModel() trainer = Trainer(gpus=2) predictions = trainer.predict(model, dm)
save_hyperparameters¶
- LightningModule.save_hyperparameters(*args, ignore=None, frame=None, logger=True)
Save arguments to
hparams
attribute.- Parameters
args¶ – single object of dict, NameSpace or OmegaConf or string names or arguments from class
__init__
ignore¶ (
Union
[Sequence
[str
],str
,None
]) – an argument name or a list of argument names from class__init__
to be ignoredframe¶ (
Optional
[FrameType
]) – a frame object. Default is Nonelogger¶ (
bool
) – Whether to send the hyperparameters to the logger. Default: True
- Return type
- Example::
>>> class ManuallyArgsModel(HyperparametersMixin): ... def __init__(self, arg1, arg2, arg3): ... super().__init__() ... # manually assign arguments ... self.save_hyperparameters('arg1', 'arg3') ... def forward(self, *args, **kwargs): ... ... >>> model = ManuallyArgsModel(1, 'abc', 3.14) >>> model.hparams "arg1": 1 "arg3": 3.14
>>> class AutomaticArgsModel(HyperparametersMixin): ... def __init__(self, arg1, arg2, arg3): ... super().__init__() ... # equivalent automatic ... self.save_hyperparameters() ... def forward(self, *args, **kwargs): ... ... >>> model = AutomaticArgsModel(1, 'abc', 3.14) >>> model.hparams "arg1": 1 "arg2": abc "arg3": 3.14
>>> class SingleArgModel(HyperparametersMixin): ... def __init__(self, params): ... super().__init__() ... # manually assign single argument ... self.save_hyperparameters(params) ... def forward(self, *args, **kwargs): ... ... >>> model = SingleArgModel(Namespace(p1=1, p2='abc', p3=3.14)) >>> model.hparams "p1": 1 "p2": abc "p3": 3.14
>>> class ManuallyArgsModel(HyperparametersMixin): ... def __init__(self, arg1, arg2, arg3): ... super().__init__() ... # pass argument(s) to ignore as a string or in a list ... self.save_hyperparameters(ignore='arg2') ... def forward(self, *args, **kwargs): ... ... >>> model = ManuallyArgsModel(1, 'abc', 3.14) >>> model.hparams "arg1": 1 "arg3": 3.14
test_step¶
- LightningModule.test_step(*args, **kwargs)[source]
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.
# the pseudocode for these calls test_outs = [] for test_batch in test_data: out = test_step(test_batch) test_outs.append(out) test_epoch_end(test_outs)
- Parameters
- Return type
- 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)
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.# CASE 2: multiple test dataloaders def test_step(self, batch, batch_idx, dataloader_idx): # 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.
test_step_end¶
- LightningModule.test_step_end(*args, **kwargs)[source]
Use this when testing with dp or ddp2 because
test_step()
will operate on only part of the batch. However, this is still optional and only needed for things like softmax or NCE loss.Note
If you later switch to ddp or some other mode, this will still be called so that you don’t have to change your code.
# pseudocode sub_batches = split_batches_for_dp(batch) batch_parts_outputs = [test_step(sub_batch) for sub_batch in sub_batches] test_step_end(batch_parts_outputs)
- Parameters
batch_parts_outputs¶ – What you return in
test_step()
for each batch part.- Return type
- Returns
None or anything
# WITHOUT test_step_end # if used in DP or DDP2, this batch is 1/num_gpus large def test_step(self, batch, batch_idx): # batch is 1/num_gpus big x, y = batch out = self(x) loss = self.softmax(out) self.log('test_loss', loss) # -------------- # with test_step_end to do softmax over the full batch def test_step(self, batch, batch_idx): # batch is 1/num_gpus big x, y = batch out = self.encoder(x) return out def test_step_end(self, output_results): # this out is now the full size of the batch all_test_step_outs = output_results.out loss = nce_loss(all_test_step_outs) self.log('test_loss', loss)
See also
See the Multi-GPU training guide for more details.
test_epoch_end¶
- LightningModule.test_epoch_end(outputs)[source]
Called at the end of a test epoch with the output of all test steps.
# the pseudocode for these calls test_outs = [] for test_batch in test_data: out = test_step(test_batch) test_outs.append(out) test_epoch_end(test_outs)
- Parameters
outputs¶ (
List
[Union
[Tensor
,Dict
[str
,Any
]]]) – List of outputs you defined intest_step_end()
, or if there are multiple dataloaders, a list containing a list of outputs for each dataloader- Return type
- Returns
None
Note
If you didn’t define a
test_step()
, this won’t be called.Examples
With a single dataloader:
def test_epoch_end(self, outputs): # do something with the outputs of all test batches all_test_preds = test_step_outputs.predictions some_result = calc_all_results(all_test_preds) self.log(some_result)
With multiple dataloaders, outputs will be a list of lists. The outer list contains one entry per dataloader, while the inner list contains the individual outputs of each test step for that dataloader.
def test_epoch_end(self, outputs): final_value = 0 for dataloader_outputs in outputs: for test_step_out in dataloader_outputs: # do something final_value += test_step_out self.log('final_metric', final_value)
to_onnx¶
- LightningModule.to_onnx(file_path, input_sample=None, **kwargs)[source]
Saves the model in ONNX format.
- Parameters
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)))
>>> with tempfile.NamedTemporaryFile(suffix='.onnx', delete=False) as tmpfile: ... model = SimpleModel() ... input_sample = torch.randn((1, 64)) ... model.to_onnx(tmpfile.name, input_sample, export_params=True) ... os.path.isfile(tmpfile.name) True
to_torchscript¶
- LightningModule.to_torchscript(file_path=None, method='script', example_inputs=None, **kwargs)[source]
By default compiles the whole model to a
ScriptModule
. If you want to use tracing, please provided the argumentmethod='trace'
and make sure that either the example_inputs argument is provided, or the model hasexample_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 (usesexample_input_array
)**kwargs¶ – Additional arguments that will be passed to the
torch.jit.script()
ortorch.jit.trace()
function.
Note
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() >>> torch.jit.save(model.to_torchscript(), "model.pt") >>> os.path.isfile("model.pt") >>> torch.jit.save(model.to_torchscript(file_path="model_trace.pt", method='trace', ... example_inputs=torch.randn(1, 64))) >>> os.path.isfile("model_trace.pt") True
training_step¶
- LightningModule.training_step(*args, **kwargs)[source]
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 yourDataLoader
. A tensor, tuple or list.optimizer_idx¶ (int) – When using multiple optimizers, this argument will also be present.
hiddens¶ (
Tensor
) – Passed in iftruncated_bptt_steps
> 0.
- Return type
- Returns
Any of.
Tensor
- The loss tensordict
- A dictionary. Can include any keys, but must include the key'loss'
None
- Training will skip to the next batch
Note
Returning
None
is currently not supported for multi-GPU or TPU, or with 16-bit precision enabled.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
If you define multiple optimizers, this step will be called with an additional
optimizer_idx
parameter.# Multiple optimizers (e.g.: GANs) def training_step(self, batch, batch_idx, optimizer_idx): if optimizer_idx == 0: # do training_step with encoder if optimizer_idx == 1: # do training_step with decoder
If you add truncated back propagation through time you will also get an additional argument with the hidden states of the previous step.
# Truncated back-propagation through time def training_step(self, batch, batch_idx, hiddens): # hiddens are the hidden states from the previous truncated backprop step ... out, hiddens = self.lstm(data, hiddens) ... return {'loss': loss, 'hiddens': hiddens}
Note
The loss value shown in the progress bar is smoothed (averaged) over the last values, so it differs from the actual loss returned in train/validation step.
training_step_end¶
- LightningModule.training_step_end(*args, **kwargs)[source]
Use this when training with dp or ddp2 because
training_step()
will operate on only part of the batch. However, this is still optional and only needed for things like softmax or NCE loss.Note
If you later switch to ddp or some other mode, this will still be called so that you don’t have to change your code
# pseudocode sub_batches = split_batches_for_dp(batch) batch_parts_outputs = [training_step(sub_batch) for sub_batch in sub_batches] training_step_end(batch_parts_outputs)
- Parameters
batch_parts_outputs¶ – What you return in training_step for each batch part.
- Return type
- Returns
Anything
When using dp/ddp2 distributed backends, only a portion of the batch is inside the training_step:
def training_step(self, batch, batch_idx): # batch is 1/num_gpus big x, y = batch out = self(x) # softmax uses only a portion of the batch in the denominator loss = self.softmax(out) loss = nce_loss(loss) return loss
If you wish to do something with all the parts of the batch, then use this method to do it:
def training_step(self, batch, batch_idx): # batch is 1/num_gpus big x, y = batch out = self.encoder(x) return {'pred': out} def training_step_end(self, training_step_outputs): gpu_0_pred = training_step_outputs[0]['pred'] gpu_1_pred = training_step_outputs[1]['pred'] gpu_n_pred = training_step_outputs[n]['pred'] # this softmax now uses the full batch loss = nce_loss([gpu_0_pred, gpu_1_pred, gpu_n_pred]) return loss
See also
See the Multi-GPU training guide for more details.
training_epoch_end¶
- LightningModule.training_epoch_end(outputs)[source]
Called at the end of the training epoch with the outputs of all training steps. Use this in case you need to do something with all the outputs returned by
training_step()
.# the pseudocode for these calls train_outs = [] for train_batch in train_data: out = training_step(train_batch) train_outs.append(out) training_epoch_end(train_outs)
- Parameters
outputs¶ (
List
[Union
[Tensor
,Dict
[str
,Any
]]]) – List of outputs you defined intraining_step()
, or if there are multiple dataloaders, a list containing a list of outputs for each dataloader.- Return type
- Returns
None
Note
If this method is not overridden, this won’t be called.
Example:
def training_epoch_end(self, training_step_outputs): # do something with all training_step outputs return result
With multiple dataloaders,
outputs
will be a list of lists. The outer list contains one entry per dataloader, while the inner list contains the individual outputs of each training step for that dataloader.def training_epoch_end(self, training_step_outputs): for out in training_step_outputs: # do something here
unfreeze¶
validation_step¶
- LightningModule.validation_step(*args, **kwargs)[source]
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.
# the pseudocode for these calls val_outs = [] for val_batch in val_data: out = validation_step(val_batch) val_outs.append(out) validation_epoch_end(val_outs)
- Parameters
- Return type
- Returns
Any object or value
None
- Validation will skip to the next batch
# pseudocode of order val_outs = [] for val_batch in val_data: out = validation_step(val_batch) if defined('validation_step_end'): out = validation_step_end(out) val_outs.append(out) val_outs = validation_epoch_end(val_outs)
# 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)
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.# CASE 2: multiple validation dataloaders def validation_step(self, batch, batch_idx, dataloader_idx): # 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.
validation_step_end¶
- LightningModule.validation_step_end(*args, **kwargs)[source]
Use this when validating with dp or ddp2 because
validation_step()
will operate on only part of the batch. However, this is still optional and only needed for things like softmax or NCE loss.Note
If you later switch to ddp or some other mode, this will still be called so that you don’t have to change your code.
# pseudocode sub_batches = split_batches_for_dp(batch) batch_parts_outputs = [validation_step(sub_batch) for sub_batch in sub_batches] validation_step_end(batch_parts_outputs)
- Parameters
batch_parts_outputs¶ – What you return in
validation_step()
for each batch part.- Return type
- Returns
None or anything
# WITHOUT validation_step_end # if used in DP or DDP2, this batch is 1/num_gpus large def validation_step(self, batch, batch_idx): # batch is 1/num_gpus big x, y = batch out = self.encoder(x) loss = self.softmax(out) loss = nce_loss(loss) self.log('val_loss', loss) # -------------- # with validation_step_end to do softmax over the full batch def validation_step(self, batch, batch_idx): # batch is 1/num_gpus big x, y = batch out = self(x) return out def validation_step_end(self, val_step_outputs): for out in val_step_outputs: # do something with these
See also
See the Multi-GPU training guide for more details.
validation_epoch_end¶
- LightningModule.validation_epoch_end(outputs)[source]
Called at the end of the validation epoch with the outputs of all validation steps.
# the pseudocode for these calls val_outs = [] for val_batch in val_data: out = validation_step(val_batch) val_outs.append(out) validation_epoch_end(val_outs)
- Parameters
outputs¶ (
List
[Union
[Tensor
,Dict
[str
,Any
]]]) – List of outputs you defined invalidation_step()
, or if there are multiple dataloaders, a list containing a list of outputs for each dataloader.- Return type
- Returns
None
Note
If you didn’t define a
validation_step()
, this won’t be called.Examples
With a single dataloader:
def validation_epoch_end(self, val_step_outputs): for out in val_step_outputs: # do something
With multiple dataloaders, outputs will be a list of lists. The outer list contains one entry per dataloader, while the inner list contains the individual outputs of each validation step for that dataloader.
def validation_epoch_end(self, outputs): for dataloader_output_result in outputs: dataloader_outs = dataloader_output_result.dataloader_i_outputs self.log('final_metric', final_value)
write_prediction¶
- LightningModule.write_prediction(name, value, filename='predictions.pt')[source]
Write predictions to disk using
torch.save
Example:
self.write_prediction('pred', torch.tensor(...), filename='my_predictions.pt')
- Parameters
Note
when running in distributed mode, calling
write_prediction
will create a file for each device with respective names:filename_rank_0.pt
,filename_rank_1.pt
, …
write_prediction_dict¶
- LightningModule.write_prediction_dict(predictions_dict, filename='predictions.pt')[source]
Write a dictonary of predictions to disk at once using
torch.save
Example:
pred_dict = {'pred1': torch.tensor(...), 'pred2': torch.tensor(...)} self.write_prediction_dict(pred_dict)
- Parameters
predictions_dict¶ (
Dict
[str
,Any
]) – dict containing predictions, where each prediction should either be singleTensor
or a list of them
Note
when running in distributed mode, calling
write_prediction_dict
will create a file for each device with respective names:filename_rank_0.pt
,filename_rank_1.pt
, …
Properties¶
These are properties available in a LightningModule.
current_epoch¶
The current epoch
def training_step(...):
if self.current_epoch == 0:
device¶
The device the module is on. Use it to keep your code device agnostic
def training_step(...):
z = torch.rand(2, 3, device=self.device)
global_rank¶
The global_rank of this LightningModule. Lightning saves logs, weights etc only from global_rank = 0. You normally do not need to use this property
Global rank refers to the index of that GPU across ALL GPUs. For example, if using 10 machines, each with 4 GPUs, the 4th GPU on the 10th machine has global_rank = 39
global_step¶
The current step (does not reset each epoch)
def training_step(...):
self.logger.experiment.log_image(..., step=self.global_step)
hparams¶
- The arguments saved by calling
save_hyperparameters
passed through__init__()
could be accessed by the
hparams
attribute.
def __init__(self, learning_rate):
self.save_hyperparameters()
def configure_optimizers(self):
return Adam(self.parameters(), lr=self.hparams.learning_rate)
logger¶
The current logger being used (tensorboard or other supported logger)
def training_step(...):
# the generic logger (same no matter if tensorboard or other supported logger)
self.logger
# the particular logger
tensorboard_logger = self.logger.experiment
local_rank¶
The local_rank of this LightningModule. Lightning saves logs, weights etc only from global_rank = 0. You normally do not need to use this property
Local rank refers to the rank on that machine. For example, if using 10 machines, the GPU at index 0 on each machine has local_rank = 0.
precision¶
The type of precision used:
def training_step(...):
if self.precision == 16:
trainer¶
Pointer to the trainer
def training_step(...):
max_steps = self.trainer.max_steps
any_flag = self.trainer.any_flag
use_amp¶
True if using Automatic Mixed Precision (AMP)
automatic_optimization¶
When set to False
, Lightning does not automate the optimization process. This means you are responsible for handling
your optimizers. However, we do take care of precision and any accelerators used.
See manual optimization for details.
def __init__(self):
self.automatic_optimization = False
def training_step(self, batch, batch_idx):
opt = self.optimizers(use_pl_optimizer=True)
loss = ...
opt.zero_grad()
self.manual_backward(loss)
opt.step()
This is recommended only if using 2+ optimizers AND if you know how to perform the optimization procedure properly. Note
that automatic optimization can still be used with multiple optimizers by relying on the optimizer_idx
parameter.
Manual optimization is most useful for research topics like reinforcement learning, sparse coding, and GAN research.
def __init__(self):
self.automatic_optimization = False
def training_step(self, batch, batch_idx):
# access your optimizers with use_pl_optimizer=False. Default is True
opt_a, opt_b = self.optimizers(use_pl_optimizer=True)
gen_loss = ...
opt_a.zero_grad()
self.manual_backward(gen_loss)
opt_a.step()
disc_loss = ...
opt_b.zero_grad()
self.manual_backward(disc_loss)
opt_b.step()
example_input_array¶
Set and access example_input_array which is basically a single batch.
def __init__(self):
self.example_input_array = ...
self.generator = ...
def on_train_epoch_end(...):
# generate some images using the example_input_array
gen_images = self.generator(self.example_input_array)
datamodule¶
Set or access your datamodule.
def configure_optimizers(self):
num_training_samples = len(self.trainer.datamodule.train_dataloader())
...
model_size¶
Get the model file size (in megabytes) using self.model_size
inside LightningModule.
truncated_bptt_steps¶
Truncated back prop breaks performs backprop every k steps of
a much longer sequence. This is made possible by passing training batches
splitted along the time-dimensions into splits of size k to the
training_step
. In order to keep the same forward propagation behavior, all
hidden states should be kept in-between each time-dimension split.
If this is enabled, your batches will automatically get truncated and the trainer will apply Truncated Backprop to it.
from pytorch_lightning import LightningModule
class MyModel(LightningModule):
def __init__(self, input_size, hidden_size, num_layers):
super().__init__()
# batch_first has to be set to True
self.lstm = nn.LSTM(
input_size=input_size,
hidden_size=hidden_size,
num_layers=num_layers,
batch_first=True,
)
...
# Important: This property activates truncated backpropagation through time
# Setting this value to 2 splits the batch into sequences of size 2
self.truncated_bptt_steps = 2
# Truncated back-propagation through time
def training_step(self, batch, batch_idx, hiddens):
x, y = batch
# the training step must be updated to accept a ``hiddens`` argument
# hiddens are the hiddens from the previous truncated backprop step
out, hiddens = self.lstm(x, hiddens)
...
return {
"loss": ...,
"hiddens": hiddens
}
Lightning takes care of splitting your batch along the time-dimension. It is
assumed to be the second dimension of your batches. Therefore, in the
example above we have set batch_first=True
.
# we use the second as the time dimension
# (batch, time, ...)
sub_batch = batch[0, 0:t, ...]
To modify how the batch is split,
override pytorch_lightning.core.LightningModule.tbptt_split_batch()
:
class LitMNIST(LightningModule):
def tbptt_split_batch(self, batch, split_size):
# do your own splitting on the batch
return splits
Hooks¶
This is the pseudocode to describe the structure of fit()
.
The inputs and outputs of each function are not represented for simplicity. Please check each function’s API reference
for more information.
def fit(...):
if global_rank == 0:
# prepare data is called on GLOBAL_ZERO only
prepare_data()
configure_callbacks()
with parallel(devices):
# devices can be GPUs, TPUs, ...
train_on_device(model)
def train_on_device(model):
# called PER DEVICE
on_fit_start()
setup('fit')
configure_optimizers()
on_pretrain_routine_start()
on_pretrain_routine_end()
# the sanity check runs here
on_train_start()
for epoch in epochs:
train_loop()
on_train_end()
on_fit_end()
teardown('fit')
def train_loop():
on_epoch_start()
on_train_epoch_start()
for batch in train_dataloader():
on_train_batch_start()
on_before_batch_transfer()
transfer_batch_to_device()
on_after_batch_transfer()
training_step()
on_before_zero_grad()
optimizer_zero_grad()
on_before_backward()
backward()
on_after_backward()
on_before_optimizer_step()
optimizer_step()
on_train_batch_end()
if should_check_val:
val_loop()
# end training epoch
training_epoch_end()
on_train_epoch_end()
on_epoch_end()
def val_loop():
on_validation_model_eval() # calls `model.eval()`
torch.set_grad_enabled(False)
on_validation_start()
on_epoch_start()
on_validation_epoch_start()
for batch in val_dataloader():
on_validation_batch_start()
on_before_batch_transfer()
transfer_batch_to_device()
on_after_batch_transfer()
validation_step()
on_validation_batch_end()
validation_epoch_end()
on_validation_epoch_end()
on_epoch_end()
on_validation_end()
# set up for train
on_validation_model_train() # calls `model.train()`
torch.set_grad_enabled(True)
backward¶
- LightningModule.backward(loss, optimizer, optimizer_idx, *args, **kwargs)[source]
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 bytraining_step()
. If gradient accumulation is used, the loss here holds the normalized value (scaled by 1 / accumulation steps).optimizer¶ (
Optional
[Optimizer
]) – Current optimizer being used.None
if using manual optimization.optimizer_idx¶ (
Optional
[int
]) – Index of the current optimizer being used.None
if using manual optimization.
- Return type
Example:
def backward(self, loss, optimizer, optimizer_idx): loss.backward()
get_progress_bar_dict¶
- LightningModule.get_progress_bar_dict()[source]
Implement this to override the default items displayed in the progress bar. By default it includes the average loss value, split index of BPTT (if used) and the version of the experiment when using a logger.
Epoch 1: 4%|▎ | 40/1095 [00:03<01:37, 10.84it/s, loss=4.501, v_num=10]
Here is an example how to override the defaults:
def get_progress_bar_dict(self): # don't show the version number items = super().get_progress_bar_dict() items.pop("v_num", None) return items
on_before_backward¶
on_after_backward¶
on_before_zero_grad¶
- ModelHooks.on_before_zero_grad(optimizer)[source]
Called after
training_step()
and beforeoptimizer.zero_grad()
.Called in the training loop after taking an optimizer step and before zeroing grads. Good place to inspect weight information with weights updated.
This is where it is called:
for optimizer in optimizers: out = training_step(...) model.on_before_zero_grad(optimizer) # < ---- called here optimizer.zero_grad() backward()
on_fit_start¶
on_fit_end¶
on_load_checkpoint¶
- CheckpointHooks.on_load_checkpoint(checkpoint)[source]
Called by Lightning to restore your model. If you saved something with
on_save_checkpoint()
this is your chance to restore this.Example:
def on_load_checkpoint(self, checkpoint): # 99% of the time you don't need to implement this method self.something_cool_i_want_to_save = checkpoint['something_cool_i_want_to_save']
Note
Lightning auto-restores global step, epoch, and train state including amp scaling. There is no need for you to restore anything regarding training.
on_save_checkpoint¶
- CheckpointHooks.on_save_checkpoint(checkpoint)[source]
Called by Lightning when saving a checkpoint to give you a chance to store anything else you might want to save.
- Parameters
checkpoint¶ (
Dict
[str
,Any
]) – The full checkpoint dictionary before it gets dumped to a file. Implementations of this hook can insert additional data into this dictionary.- Return type
Example:
def on_save_checkpoint(self, checkpoint): # 99% of use cases you don't need to implement this method checkpoint['something_cool_i_want_to_save'] = my_cool_pickable_object
Note
Lightning saves all aspects of training (epoch, global step, etc…) including amp scaling. There is no need for you to store anything about training.
on_train_start¶
on_train_end¶
on_validation_start¶
on_validation_end¶
on_pretrain_routine_start¶
on_pretrain_routine_end¶
on_test_batch_start¶
- ModelHooks.on_test_batch_start(batch, batch_idx, dataloader_idx)[source]
Called in the test loop before anything happens for that batch.
on_test_batch_end¶
- ModelHooks.on_test_batch_end(outputs, batch, batch_idx, dataloader_idx)[source]
Called in the test loop after the batch.
on_test_epoch_start¶
on_test_epoch_end¶
on_test_start¶
on_test_end¶
on_train_batch_start¶
- ModelHooks.on_train_batch_start(batch, batch_idx, dataloader_idx)[source]
Called in the training loop before anything happens for that batch.
If you return -1 here, you will skip training for the rest of the current epoch.
on_train_batch_end¶
- ModelHooks.on_train_batch_end(outputs, batch, batch_idx, dataloader_idx)[source]
Called in the training loop after the batch.
on_epoch_start¶
on_epoch_end¶
on_train_epoch_start¶
on_train_epoch_end¶
- ModelHooks.on_train_epoch_end(unused=None)[source]
Called in the training loop at the very end of the epoch.
To access all batch outputs at the end of the epoch, either:
Implement training_epoch_end in the LightningModule OR
Cache data across steps on the attribute(s) of the LightningModule and access them in this hook
on_validation_batch_start¶
- ModelHooks.on_validation_batch_start(batch, batch_idx, dataloader_idx)[source]
Called in the validation loop before anything happens for that batch.
on_validation_batch_end¶
- ModelHooks.on_validation_batch_end(outputs, batch, batch_idx, dataloader_idx)[source]
Called in the validation loop after the batch.
- Parameters
- Return type
on_validation_epoch_start¶
on_validation_epoch_end¶
on_post_move_to_device¶
- ModelHooks.on_post_move_to_device()[source]
Called in the
parameter_validation
decorator afterto()
is called. This is a good place to tie weights between modules after moving them to a device. Can be used when training models with weight sharing properties on TPU.Addresses the handling of shared weights on TPU: https://github.com/pytorch/xla/blob/master/TROUBLESHOOTING.md#xla-tensor-quirks
Example:
def on_post_move_to_device(self): self.decoder.weight = self.encoder.weight
- Return type
on_validation_model_eval¶
on_validation_model_train¶
on_test_model_eval¶
on_test_model_train¶
on_before_optimizer_step¶
- ModelHooks.on_before_optimizer_step(optimizer, optimizer_idx)[source]
Called before
optimizer.step()
.The hook is only called if gradients do not need to be accumulated. See:
accumulate_grad_batches
. If using native AMP, the loss will be unscaled before calling this hook. See these docs for more information on the scaling of gradients.- Parameters
- Return type
Example:
def on_before_optimizer_step(self, optimizer, optimizer_idx): # example to inspect gradient information in tensorboard if self.trainer.global_step % 25 == 0: # don't make the tf file huge for k, v in self.named_parameters(): self.logger.experiment.add_histogram( tag=k, values=v.grad, global_step=self.trainer.global_step )
optimizer_step¶
- LightningModule.optimizer_step(epoch=None, batch_idx=None, optimizer=None, optimizer_idx=None, optimizer_closure=None, on_tpu=None, using_native_amp=None, using_lbfgs=None)[source]
Override this method to adjust the default way the
Trainer
calls each optimizer. By default, Lightning callsstep()
andzero_grad()
as shown in the example once per optimizer. This method (andzero_grad()
) won’t be called during the accumulation phase whenTrainer(accumulate_grad_batches != 1)
.Warning
If you are overriding this method, make sure that you pass the
optimizer_closure
parameter tooptimizer.step()
function as shown in the examples. This ensures thattraining_step()
,optimizer.zero_grad()
,backward()
are called within the training loop.- Parameters
optimizer_idx¶ (
Optional
[int
]) – If you used multiple optimizers, this indexes into that list.optimizer_closure¶ (
Optional
[Callable
]) – Closure for all optimizersusing_native_amp¶ (
Optional
[bool
]) –True
if using native ampusing_lbfgs¶ (
Optional
[bool
]) – True if the matching optimizer istorch.optim.LBFGS
- Return type
Examples:
# DEFAULT def optimizer_step(self, epoch, batch_idx, optimizer, optimizer_idx, optimizer_closure, on_tpu, using_native_amp, using_lbfgs): optimizer.step(closure=optimizer_closure) # Alternating schedule for optimizer steps (i.e.: GANs) def optimizer_step(self, epoch, batch_idx, optimizer, optimizer_idx, optimizer_closure, on_tpu, using_native_amp, using_lbfgs): # update generator opt every step if optimizer_idx == 0: optimizer.step(closure=optimizer_closure) # update discriminator opt every 2 steps if optimizer_idx == 1: if (batch_idx + 1) % 2 == 0 : optimizer.step(closure=optimizer_closure) # ... # add as many optimizers as you want
Here’s another example showing how to use this for more advanced things such as learning rate warm-up:
# learning rate warm-up def optimizer_step(self, epoch, batch_idx, optimizer, optimizer_idx, optimizer_closure, on_tpu, using_native_amp, using_lbfgs): # warm up lr if self.trainer.global_step < 500: lr_scale = min(1., float(self.trainer.global_step + 1) / 500.) for pg in optimizer.param_groups: pg['lr'] = lr_scale * self.learning_rate # update params optimizer.step(closure=optimizer_closure)
optimizer_zero_grad¶
- LightningModule.optimizer_zero_grad(epoch, batch_idx, optimizer, optimizer_idx)[source]
Override this method to change the default behaviour of
optimizer.zero_grad()
.- Parameters
Examples:
# DEFAULT def optimizer_zero_grad(self, epoch, batch_idx, optimizer, optimizer_idx): optimizer.zero_grad() # Set gradients to `None` instead of zero to improve performance. def optimizer_zero_grad(self, epoch, batch_idx, optimizer, optimizer_idx): optimizer.zero_grad(set_to_none=True)
See
torch.optim.Optimizer.zero_grad()
for the explanation of the above example.
prepare_data¶
- LightningModule.prepare_data()
Use this to download and prepare data. :rtype:
None
Warning
DO NOT set state to the model (use setup instead) since this is NOT called on every GPU in DDP/TPU
Example:
def prepare_data(self): # good download_data() tokenize() etc() # bad self.split = data_split self.some_state = some_other_state()
In DDP prepare_data can be called in two ways (using Trainer(prepare_data_per_node)):
Once per node. This is the default and is only called on LOCAL_RANK=0.
Once in total. Only called on GLOBAL_RANK=0.
Example:
# DEFAULT # called once per node on LOCAL_RANK=0 of that node Trainer(prepare_data_per_node=True) # call on GLOBAL_RANK=0 (great for shared file systems) Trainer(prepare_data_per_node=False)
This is called before requesting the dataloaders:
model.prepare_data() if ddp/tpu: init() model.setup(stage) model.train_dataloader() model.val_dataloader() model.test_dataloader()
setup¶
- DataHooks.setup(stage=None)[source]
Called at the beginning of fit (train + validate), validate, test, and predict. This is a good hook when you need to build models dynamically or adjust something about them. This hook is called on every process when using DDP.
Example:
class LitModel(...): def __init__(self): self.l1 = None def prepare_data(self): download_data() tokenize() # don't do this self.something = else def setup(stage): data = Load_data(...) self.l1 = nn.Linear(28, data.num_classes)
tbptt_split_batch¶
- LightningModule.tbptt_split_batch(batch, split_size)[source]
When using truncated backpropagation through time, each batch must be split along the time dimension. Lightning handles this by default, but for custom behavior override this function.
- Parameters
- Return type
- Returns
List of batch splits. Each split will be passed to
training_step()
to enable truncated back propagation through time. The default implementation splits root level Tensors and Sequences at dim=1 (i.e. time dim). It assumes that each time dim is the same length.
Examples:
def tbptt_split_batch(self, batch, split_size): splits = [] for t in range(0, time_dims[0], split_size): batch_split = [] for i, x in enumerate(batch): if isinstance(x, torch.Tensor): split_x = x[:, t:t + split_size] elif isinstance(x, collections.Sequence): split_x = [None] * len(x) for batch_idx in range(len(x)): split_x[batch_idx] = x[batch_idx][t:t + split_size] batch_split.append(split_x) splits.append(batch_split) return splits
Note
Called in the training loop after
on_batch_start()
iftruncated_bptt_steps
> 0. Each returned batch split is passed separately totraining_step()
.
teardown¶
train_dataloader¶
- DataHooks.train_dataloader()[source]
Implement one or more PyTorch DataLoaders for training.
- Return type
Union
[DataLoader
,Sequence
[DataLoader
],Sequence
[Sequence
[DataLoader
]],Sequence
[Dict
[str
,DataLoader
]],Dict
[str
,DataLoader
],Dict
[str
,Dict
[str
,DataLoader
]],Dict
[str
,Sequence
[DataLoader
]]]- Returns
A collection of
torch.utils.data.DataLoader
specifying training samples. In the case of multiple dataloaders, please see this page.
The dataloader you return will not be reloaded unless you set
reload_dataloaders_every_n_epochs
to a positive integer.For data processing use the following pattern:
download in
prepare_data()
process and split in
setup()
However, the above are only necessary for distributed processing.
Warning
do not assign state in prepare_data
fit()
…
Note
Lightning adds the correct sampler for distributed and arbitrary hardware. There is no need to set it yourself.
Example:
# single dataloader def train_dataloader(self): transform = transforms.Compose([transforms.ToTensor(), transforms.Normalize((0.5,), (1.0,))]) dataset = MNIST(root='/path/to/mnist/', train=True, transform=transform, download=True) loader = torch.utils.data.DataLoader( dataset=dataset, batch_size=self.batch_size, shuffle=True ) return loader # multiple dataloaders, return as list def train_dataloader(self): mnist = MNIST(...) cifar = CIFAR(...) mnist_loader = torch.utils.data.DataLoader( dataset=mnist, batch_size=self.batch_size, shuffle=True ) cifar_loader = torch.utils.data.DataLoader( dataset=cifar, batch_size=self.batch_size, shuffle=True ) # each batch will be a list of tensors: [batch_mnist, batch_cifar] return [mnist_loader, cifar_loader] # multiple dataloader, return as dict def train_dataloader(self): mnist = MNIST(...) cifar = CIFAR(...) mnist_loader = torch.utils.data.DataLoader( dataset=mnist, batch_size=self.batch_size, shuffle=True ) cifar_loader = torch.utils.data.DataLoader( dataset=cifar, batch_size=self.batch_size, shuffle=True ) # each batch will be a dict of tensors: {'mnist': batch_mnist, 'cifar': batch_cifar} return {'mnist': mnist_loader, 'cifar': cifar_loader}
val_dataloader¶
- DataHooks.val_dataloader()[source]
Implement one or multiple PyTorch DataLoaders for validation.
The dataloader you return will not be reloaded unless you set
reload_dataloaders_every_n_epochs
to a positive integer.It’s recommended that all data downloads and preparation happen in
prepare_data()
.Note
Lightning adds the correct sampler for distributed and arbitrary hardware There is no need to set it yourself.
- Return type
- Returns
A
torch.utils.data.DataLoader
or a sequence of them specifying validation samples.
Examples:
def val_dataloader(self): transform = transforms.Compose([transforms.ToTensor(), transforms.Normalize((0.5,), (1.0,))]) dataset = MNIST(root='/path/to/mnist/', train=False, transform=transform, download=True) loader = torch.utils.data.DataLoader( dataset=dataset, batch_size=self.batch_size, shuffle=False ) return loader # can also return multiple dataloaders def val_dataloader(self): return [loader_a, loader_b, ..., loader_n]
Note
If you don’t need a validation dataset and a
validation_step()
, you don’t need to implement this method.Note
In the case where you return multiple validation dataloaders, the
validation_step()
will have an argumentdataloader_idx
which matches the order here.
test_dataloader¶
- DataHooks.test_dataloader()[source]
Implement one or multiple PyTorch DataLoaders for testing.
The dataloader you return will not be reloaded unless you set
reload_dataloaders_every_n_epochs
to a postive integer.For data processing use the following pattern:
download in
prepare_data()
process and split in
setup()
However, the above are only necessary for distributed processing.
Warning
do not assign state in prepare_data
Note
Lightning adds the correct sampler for distributed and arbitrary hardware. There is no need to set it yourself.
- Return type
- Returns
A
torch.utils.data.DataLoader
or a sequence of them specifying testing samples.
Example:
def test_dataloader(self): transform = transforms.Compose([transforms.ToTensor(), transforms.Normalize((0.5,), (1.0,))]) dataset = MNIST(root='/path/to/mnist/', train=False, transform=transform, download=True) loader = torch.utils.data.DataLoader( dataset=dataset, batch_size=self.batch_size, shuffle=False ) return loader # can also return multiple dataloaders def test_dataloader(self): return [loader_a, loader_b, ..., loader_n]
Note
If you don’t need a test dataset and a
test_step()
, you don’t need to implement this method.Note
In the case where you return multiple test dataloaders, the
test_step()
will have an argumentdataloader_idx
which matches the order here.
transfer_batch_to_device¶
- DataHooks.transfer_batch_to_device(batch, device, dataloader_idx)[source]
Override this hook if your
DataLoader
returns tensors wrapped in a custom data structure.The data types listed below (and any arbitrary nesting of them) are supported out of the box:
torch.Tensor
or anything that implements .to(…)torchtext.data.batch.Batch
For anything else, you need to define how the data is moved to the target device (CPU, GPU, TPU, …).
Note
This hook should only transfer the data and not modify it, nor should it move the data to any other device than the one passed in as argument (unless you know what you are doing). To check the current state of execution of this hook you can use
self.trainer.training/testing/validating/predicting
so that you can add different logic as per your requirement.Note
This hook only runs on single GPU training and DDP (no data-parallel). Data-Parallel support will come in near future.
- Parameters
- Return type
- Returns
A reference to the data on the new device.
Example:
def transfer_batch_to_device(self, batch, device): if isinstance(batch, CustomBatch): # move all tensors in your custom data structure to the device batch.samples = batch.samples.to(device) batch.targets = batch.targets.to(device) else: batch = super().transfer_batch_to_device(data, device) return batch
- Raises
MisconfigurationException – If using data-parallel,
Trainer(accelerator='dp')
.
See also
move_data_to_device()
apply_to_collection()
on_before_batch_transfer¶
- DataHooks.on_before_batch_transfer(batch, dataloader_idx)[source]
Override to alter or apply batch augmentations to your batch before it is transferred to the device.
Note
To check the current state of execution of this hook you can use
self.trainer.training/testing/validating/predicting
so that you can add different logic as per your requirement.Note
This hook only runs on single GPU training and DDP (no data-parallel). Data-Parallel support will come in near future.
- Parameters
- Return type
- Returns
A batch of data
Example:
def on_before_batch_transfer(self, batch, dataloader_idx): batch['x'] = transforms(batch['x']) return batch
- Raises
MisconfigurationException – If using data-parallel,
Trainer(accelerator='dp')
.
on_after_batch_transfer¶
- DataHooks.on_after_batch_transfer(batch, dataloader_idx)[source]
Override to alter or apply batch augmentations to your batch after it is transferred to the device.
Note
To check the current state of execution of this hook you can use
self.trainer.training/testing/validating/predicting
so that you can add different logic as per your requirement.Note
This hook only runs on single GPU training and DDP (no data-parallel). Data-Parallel support will come in near future.
- Parameters
- Return type
- Returns
A batch of data
Example:
def on_after_batch_transfer(self, batch, dataloader_idx): batch['x'] = gpu_transforms(batch['x']) return batch
- Raises
MisconfigurationException – If using data-parallel,
Trainer(accelerator='dp')
.