.. testsetup:: *

    import os
    from pytorch_lightning.trainer.trainer import Trainer
    from pytorch_lightning.core.lightning import LightningModule
    from pytorch_lightning.utilities.seed import seed_everything

Trainer

Once you've organized your PyTorch code into a LightningModule, the Trainer automates everything else.

This abstraction achieves the following:

You maintain control over all aspects via PyTorch code without an added abstraction.
The trainer uses best practices embedded by contributors and users from top AI labs such as Facebook AI Research, NYU, MIT, Stanford, etc...
The trainer allows overriding any key part that you don't want automated.

Basic use

This is the basic use of the trainer:

model = MyLightningModule()

trainer = Trainer()
trainer.fit(model, train_dataloader, val_dataloader)

Under the hood

Under the hood, the Lightning Trainer handles the training loop details for you, some examples include:

Automatically enabling/disabling grads
Running the training, validation and test dataloaders
Calling the Callbacks at the appropriate times
Putting batches and computations on the correct devices

Here's the pseudocode for what the trainer does under the hood (showing the train loop only)

# put model in train mode
model.train()
torch.set_grad_enabled(True)

losses = []
for batch in train_dataloader:
    # calls hooks like this one
    on_train_batch_start()

    # train step
    loss = training_step(batch)

    # clear gradients
    optimizer.zero_grad()

    # backward
    loss.backward()

    # update parameters
    optimizer.step()

    losses.append(loss)

Trainer in Python scripts

In Python scripts, it's recommended you use a main function to call the Trainer.

from argparse import ArgumentParser


def main(hparams):
    model = LightningModule()
    trainer = Trainer(gpus=hparams.gpus)
    trainer.fit(model)


if __name__ == "__main__":
    parser = ArgumentParser()
    parser.add_argument("--gpus", default=None)
    args = parser.parse_args()

    main(args)

So you can run it like so:

python main.py --gpus 2

Note

Pro-tip: You don't need to define all flags manually. Lightning can add them automatically

from argparse import ArgumentParser


def main(args):
    model = LightningModule()
    trainer = Trainer.from_argparse_args(args)
    trainer.fit(model)


if __name__ == "__main__":
    parser = ArgumentParser()
    parser = Trainer.add_argparse_args(parser)
    args = parser.parse_args()

    main(args)

So you can run it like so:

python main.py --gpus 2 --max_steps 10 --limit_train_batches 10 --any_trainer_arg x

Note

If you want to stop a training run early, you can press "Ctrl + C" on your keyboard. The trainer will catch the KeyboardInterrupt and attempt a graceful shutdown, including running accelerator callback on_train_end to clean up memory. The trainer object will also set an attribute interrupted to True in such cases. If you have a callback which shuts down compute resources, for example, you can conditionally run the shutdown logic for only uninterrupted runs.

Validation

You can perform an evaluation epoch over the validation set, outside of the training loop, using :meth:`pytorch_lightning.trainer.trainer.Trainer.validate`. This might be useful if you want to collect new metrics from a model right at its initialization or after it has already been trained.

trainer.validate(dataloaders=val_dataloaders)

Testing

Once you're done training, feel free to run the test set! (Only right before publishing your paper or pushing to production)

trainer.test(dataloaders=test_dataloaders)

Reproducibility

To ensure full reproducibility from run to run you need to set seeds for pseudo-random generators, and set deterministic flag in Trainer.

Example:

from pytorch_lightning import Trainer, seed_everything

seed_everything(42, workers=True)
# sets seeds for numpy, torch, python.random and PYTHONHASHSEED.
model = Model()
trainer = Trainer(deterministic=True)

By setting workers=True in :func:`~pytorch_lightning.utilities.seed.seed_everything`, Lightning derives unique seeds across all dataloader workers and processes for :mod:`torch`, :mod:`numpy` and stdlib :mod:`random` number generators. When turned on, it ensures that e.g. data augmentations are not repeated across workers.

Trainer flags

accelerator

Supports passing different accelerator types ("cpu", "gpu", "tpu", "ipu", "auto") as well as custom accelerator instances.

# CPU accelerator
trainer = Trainer(accelerator="cpu")

# Training with GPU Accelerator using 2 gpus
trainer = Trainer(devices=2, accelerator="gpu")

# Training with TPU Accelerator using 8 tpu cores
trainer = Trainer(devices=8, accelerator="tpu")

# Training with GPU Accelerator using the DistributedDataParallel strategy
trainer = Trainer(devices=4, accelerator="gpu", strategy="ddp")

Note

The "auto" option recognizes the machine you are on, and selects the respective Accelerator.

# If your machine has GPUs, it will use the GPU Accelerator for training
trainer = Trainer(devices=2, accelerator="auto")

You can also modify hardware behavior by subclassing an existing accelerator to adjust for your needs.

Example:

class MyOwnAcc(CPUAccelerator):
    ...

Trainer(accelerator=MyOwnAcc())

Warning

Passing training strategies (e.g., "ddp") to accelerator has been deprecated in v1.5.0 and will be removed in v1.7.0. Please use the strategy argument instead.

accumulate_grad_batches

Accumulates grads every k batches or as set up in the dict. Trainer also calls optimizer.step() for the last indivisible step number.

.. testcode::

    # default used by the Trainer (no accumulation)
    trainer = Trainer(accumulate_grad_batches=1)

Example:

# accumulate every 4 batches (effective batch size is batch*4)
trainer = Trainer(accumulate_grad_batches=4)

# no accumulation for epochs 1-4. accumulate 3 for epochs 5-10. accumulate 20 after that
trainer = Trainer(accumulate_grad_batches={5: 3, 10: 20})

amp_backend

Use PyTorch AMP ('native'), or NVIDIA apex ('apex').

.. testcode::

    # using PyTorch built-in AMP, default used by the Trainer
    trainer = Trainer(amp_backend="native")

    # using NVIDIA Apex
    trainer = Trainer(amp_backend="apex")

amp_level

The optimization level to use (O1, O2, etc...) for 16-bit GPU precision (using NVIDIA apex under the hood).

Check NVIDIA apex docs for level

Example:

# default used by the Trainer
trainer = Trainer(amp_level='O2')

auto_scale_batch_size

Automatically tries to find the largest batch size that fits into memory, before any training.

# default used by the Trainer (no scaling of batch size)
trainer = Trainer(auto_scale_batch_size=None)

# run batch size scaling, result overrides hparams.batch_size
trainer = Trainer(auto_scale_batch_size="binsearch")

# call tune to find the batch size
trainer.tune(model)

auto_select_gpus

If enabled and gpus is an integer, pick available gpus automatically. This is especially useful when GPUs are configured to be in "exclusive mode", such that only one process at a time can access them.

Example:

# no auto selection (picks first 2 gpus on system, may fail if other process is occupying)
trainer = Trainer(gpus=2, auto_select_gpus=False)

# enable auto selection (will find two available gpus on system)
trainer = Trainer(gpus=2, auto_select_gpus=True)

# specifies all GPUs regardless of its availability
Trainer(gpus=-1, auto_select_gpus=False)

# specifies all available GPUs (if only one GPU is not occupied, uses one gpu)
Trainer(gpus=-1, auto_select_gpus=True)

auto_lr_find

Runs a learning rate finder algorithm (see this paper) when calling trainer.tune(), to find optimal initial learning rate.

# default used by the Trainer (no learning rate finder)
trainer = Trainer(auto_lr_find=False)

Example:

# run learning rate finder, results override hparams.learning_rate
trainer = Trainer(auto_lr_find=True)

# call tune to find the lr
trainer.tune(model)

Example:

# run learning rate finder, results override hparams.my_lr_arg
trainer = Trainer(auto_lr_find='my_lr_arg')

# call tune to find the lr
trainer.tune(model)

Note

See the :doc:`learning rate finder guide <../advanced/lr_finder>`.

benchmark

If true enables cudnn.benchmark. This flag is likely to increase the speed of your system if your input sizes don't change. However, if it does, then it will likely make your system slower.

The speedup comes from allowing the cudnn auto-tuner to find the best algorithm for the hardware [see discussion here].

Example:

# default used by the Trainer
trainer = Trainer(benchmark=False)

deterministic

If true enables cudnn.deterministic. Might make your system slower, but ensures reproducibility. Also sets $HOROVOD_FUSION_THRESHOLD=0.

For more info check [pytorch docs].

Example:

# default used by the Trainer
trainer = Trainer(deterministic=False)

callbacks

Add a list of :class:`~pytorch_lightning.callbacks.Callback`. Callbacks run sequentially in the order defined here with the exception of :class:`~pytorch_lightning.callbacks.model_checkpoint.ModelCheckpoint` callbacks which run after all others to ensure all states are saved to the checkpoints.

# a list of callbacks
callbacks = [PrintCallback()]
trainer = Trainer(callbacks=callbacks)

Example:

from pytorch_lightning.callbacks import Callback

class PrintCallback(Callback):
    def on_train_start(self, trainer, pl_module):
        print("Training is started!")
    def on_train_end(self, trainer, pl_module):
        print("Training is done.")

Model-specific callbacks can also be added inside the LightningModule through :meth:`~pytorch_lightning.core.lightning.LightningModule.configure_callbacks`. Callbacks returned in this hook will extend the list initially given to the Trainer argument, and replace the trainer callbacks should there be two or more of the same type. :class:`~pytorch_lightning.callbacks.model_checkpoint.ModelCheckpoint` callbacks always run last.

check_val_every_n_epoch

Check val every n train epochs.

Example:

# default used by the Trainer
trainer = Trainer(check_val_every_n_epoch=1)

# run val loop every 10 training epochs
trainer = Trainer(check_val_every_n_epoch=10)

checkpoint_callback

Warning

checkpoint_callback has been deprecated in v1.5 and will be removed in v1.7. To disable checkpointing, pass enable_checkpointing = False to the Trainer instead.

default_root_dir

Default path for logs and weights when no logger or :class:`pytorch_lightning.callbacks.ModelCheckpoint` callback passed. On certain clusters you might want to separate where logs and checkpoints are stored. If you don't then use this argument for convenience. Paths can be local paths or remote paths such as s3://bucket/path or 'hdfs://path/'. Credentials will need to be set up to use remote filepaths.

.. testcode::

    # default used by the Trainer
    trainer = Trainer(default_root_dir=os.getcwd())

devices

Number of devices to train on (int), which devices to train on (list or str), or "auto". It will be mapped to either gpus, tpu_cores, num_processes or ipus, based on the accelerator type ("cpu", "gpu", "tpu", "ipu", "auto").

# Training with CPU Accelerator using 2 processes
trainer = Trainer(devices=2, accelerator="cpu")

# Training with GPU Accelerator using GPUs 1 and 3
trainer = Trainer(devices=[1, 3], accelerator="gpu")

# Training with TPU Accelerator using 8 tpu cores
trainer = Trainer(devices=8, accelerator="tpu")

Tip

The "auto" option recognizes the devices to train on, depending on the Accelerator being used.

# If your machine has GPUs, it will use all the available GPUs for training
trainer = Trainer(devices="auto", accelerator="auto")

# Training with CPU Accelerator using 1 process
trainer = Trainer(devices="auto", accelerator="cpu")

# Training with TPU Accelerator using 8 tpu cores
trainer = Trainer(devices="auto", accelerator="tpu")

# Training with IPU Accelerator using 4 ipus
trainer = Trainer(devices="auto", accelerator="ipu")

enable_checkpointing

By default Lightning saves a checkpoint for you in your current working directory, with the state of your last training epoch, Checkpoints capture the exact value of all parameters used by a model. To disable automatic checkpointing, set this to False.

# default used by Trainer, saves the most recent model to a single checkpoint after each epoch
trainer = Trainer(enable_checkpointing=True)

# turn off automatic checkpointing
trainer = Trainer(enable_checkpointing=False)

You can override the default behavior by initializing the :class:`~pytorch_lightning.callbacks.ModelCheckpoint` callback, and adding it to the :paramref:`~pytorch_lightning.trainer.trainer.Trainer.callbacks` list. See :doc:`Saving and Loading Weights <../common/weights_loading>` for how to customize checkpointing.

.. testcode::

    from pytorch_lightning.callbacks import ModelCheckpoint

    # Init ModelCheckpoint callback, monitoring 'val_loss'
    checkpoint_callback = ModelCheckpoint(monitor="val_loss")

    # Add your callback to the callbacks list
    trainer = Trainer(callbacks=[checkpoint_callback])

fast_dev_run

Runs n if set to n (int) else 1 if set to True batch(es) of train, val and test to find any bugs (ie: a sort of unit test).

Under the hood the pseudocode looks like this when running fast_dev_run with a single batch:

# loading
__init__()
prepare_data

# test training step
training_batch = next(train_dataloader)
training_step(training_batch)

# test val step
val_batch = next(val_dataloader)
out = validation_step(val_batch)
validation_epoch_end([out])

.. testcode::

    # default used by the Trainer
    trainer = Trainer(fast_dev_run=False)

    # runs 1 train, val, test batch and program ends
    trainer = Trainer(fast_dev_run=True)

    # runs 7 train, val, test batches and program ends
    trainer = Trainer(fast_dev_run=7)

Note

This argument is a bit different from limit_train/val/test_batches. Setting this argument will disable tuner, checkpoint callbacks, early stopping callbacks, loggers and logger callbacks like LearningRateLogger and runs for only 1 epoch. This must be used only for debugging purposes. limit_train/val/test_batches only limits the number of batches and won't disable anything.

flush_logs_every_n_steps

Warning

flush_logs_every_n_steps has been deprecated in v1.5 and will be removed in v1.7. Please configure flushing directly in the logger instead.

Writes logs to disk this often.

.. testcode::

    # default used by the Trainer
    trainer = Trainer(flush_logs_every_n_steps=100)

See Also:

:doc:`logging <../extensions/logging>`

gpus

Number of GPUs to train on (int)
or which GPUs to train on (list)
can handle strings

.. testcode::

    # default used by the Trainer (ie: train on CPU)
    trainer = Trainer(gpus=None)

    # equivalent
    trainer = Trainer(gpus=0)

Example:

# int: train on 2 gpus
trainer = Trainer(gpus=2)

# list: train on GPUs 1, 4 (by bus ordering)
trainer = Trainer(gpus=[1, 4])
trainer = Trainer(gpus='1, 4') # equivalent

# -1: train on all gpus
trainer = Trainer(gpus=-1)
trainer = Trainer(gpus='-1') # equivalent

# combine with num_nodes to train on multiple GPUs across nodes
# uses 8 gpus in total
trainer = Trainer(gpus=2, num_nodes=4)

# train only on GPUs 1 and 4 across nodes
trainer = Trainer(gpus=[1, 4], num_nodes=4)

See Also:

:doc:`Multi-GPU training guide <../advanced/multi_gpu>`.

gradient_clip_val

Gradient clipping value

0 means don't clip.

.. testcode::

    # default used by the Trainer
    trainer = Trainer(gradient_clip_val=0.0)

limit_train_batches

How much of training dataset to check. Useful when debugging or testing something that happens at the end of an epoch.

.. testcode::

    # default used by the Trainer
    trainer = Trainer(limit_train_batches=1.0)

Example:

# default used by the Trainer
trainer = Trainer(limit_train_batches=1.0)

# run through only 25% of the training set each epoch
trainer = Trainer(limit_train_batches=0.25)

# run through only 10 batches of the training set each epoch
trainer = Trainer(limit_train_batches=10)

limit_test_batches

How much of test dataset to check.

.. testcode::

    # default used by the Trainer
    trainer = Trainer(limit_test_batches=1.0)

    # run through only 25% of the test set each epoch
    trainer = Trainer(limit_test_batches=0.25)

    # run for only 10 batches
    trainer = Trainer(limit_test_batches=10)

In the case of multiple test dataloaders, the limit applies to each dataloader individually.

limit_val_batches

How much of validation dataset to check. Useful when debugging or testing something that happens at the end of an epoch.

.. testcode::

    # default used by the Trainer
    trainer = Trainer(limit_val_batches=1.0)

    # run through only 25% of the validation set each epoch
    trainer = Trainer(limit_val_batches=0.25)

    # run for only 10 batches
    trainer = Trainer(limit_val_batches=10)

In the case of multiple validation dataloaders, the limit applies to each dataloader individually.

log_every_n_steps

How often to add logging rows (does not write to disk)

.. testcode::

    # default used by the Trainer
    trainer = Trainer(log_every_n_steps=50)

See Also:

:doc:`logging <../extensions/logging>`

logger

:doc:`Logger <../common/loggers>` (or iterable collection of loggers) for experiment tracking. A True value uses the default TensorBoardLogger shown below. False will disable logging.

.. testcode::

    from pytorch_lightning.loggers import TensorBoardLogger

    # default logger used by trainer
    logger = TensorBoardLogger(save_dir=os.getcwd(), version=1, name="lightning_logs")
    Trainer(logger=logger)

max_epochs

Stop training once this number of epochs is reached

.. testcode::

    # default used by the Trainer
    trainer = Trainer(max_epochs=1000)

If both max_epochs and max_steps aren't specified, max_epochs will default to 1000 and a UserWarning will be displayed stating the max_epochs is not set and defaulted to 1000. To enable infinite training, set max_epochs = -1.

min_epochs

Force training for at least these many epochs

.. testcode::

    # default used by the Trainer
    trainer = Trainer(min_epochs=1)

max_steps

Stop training after this number of steps Training will stop if max_steps or max_epochs have reached (earliest).

.. testcode::

    # Default (disabled)
    trainer = Trainer(max_steps=None)

    # Stop after 100 steps
    trainer = Trainer(max_steps=100)

If max_steps is not specified, max_epochs will be used instead (and max_epochs defaults to 1000 if max_epochs is not specified). To disable this default, set max_steps = -1.

min_steps

Force training for at least these number of steps. Trainer will train model for at least min_steps or min_epochs (latest).

.. testcode::

    # Default (disabled)
    trainer = Trainer(min_steps=None)

    # Run at least for 100 steps (disable min_epochs)
    trainer = Trainer(min_steps=100, min_epochs=0)

max_time

Set the maximum amount of time for training. Training will get interrupted mid-epoch. For customizable options use the :class:`~pytorch_lightning.callbacks.timer.Timer` callback.

.. testcode::

    # Default (disabled)
    trainer = Trainer(max_time=None)

    # Stop after 12 hours of training or when reaching 10 epochs (string)
    trainer = Trainer(max_time="00:12:00:00", max_epochs=10)

    # Stop after 1 day and 5 hours (dict)
    trainer = Trainer(max_time={"days": 1, "hours": 5})

In case max_time is used together with min_steps or min_epochs, the min_* requirement always has precedence.

num_nodes

Number of GPU nodes for distributed training.

.. testcode::

    # default used by the Trainer
    trainer = Trainer(num_nodes=1)

    # to train on 8 nodes
    trainer = Trainer(num_nodes=8)

num_processes

Number of processes to train with. Automatically set to the number of GPUs when using strategy="ddp". Set to a number greater than 1 when using accelerator="cpu" and strategy="ddp" to mimic distributed training on a machine without GPUs. This is useful for debugging, but will not provide any speedup, since single-process Torch already makes efficient use of multiple CPUs. While it would typically spawns subprocesses for training, setting num_nodes > 1 and keeping num_processes = 1 runs training in the main process.

.. testcode::

    # Simulate DDP for debugging on your GPU-less laptop
    trainer = Trainer(accelerator="cpu", strategy="ddp", num_processes=2)

num_sanity_val_steps

Sanity check runs n batches of val before starting the training routine. This catches any bugs in your validation without having to wait for the first validation check. The Trainer uses 2 steps by default. Turn it off or modify it here.

.. testcode::

    # default used by the Trainer
    trainer = Trainer(num_sanity_val_steps=2)

    # turn it off
    trainer = Trainer(num_sanity_val_steps=0)

    # check all validation data
    trainer = Trainer(num_sanity_val_steps=-1)

This option will reset the validation dataloader unless num_sanity_val_steps=0.

overfit_batches

Uses this much data of the training set. If nonzero, will use the same training set for validation and testing. If the training dataloaders have shuffle=True, Lightning will automatically disable it.

Useful for quickly debugging or trying to overfit on purpose.

.. testcode::

    # default used by the Trainer
    trainer = Trainer(overfit_batches=0.0)

    # use only 1% of the train set (and use the train set for val and test)
    trainer = Trainer(overfit_batches=0.01)

    # overfit on 10 of the same batches
    trainer = Trainer(overfit_batches=10)

plugins

:ref:`Plugins` allow you to connect arbitrary backends, precision libraries, clusters etc. For example:

:ref:`DDP <multi_gpu>`
TorchElastic
:ref:`Apex <amp>`

To define your own behavior, subclass the relevant class and pass it in. Here's an example linking up your own :class:`~pytorch_lightning.plugins.environments.ClusterEnvironment`.

from pytorch_lightning.plugins.environments import ClusterEnvironment


class MyCluster(ClusterEnvironment):
    def main_address(self):
        return your_main_address

    def main_port(self):
        return your_main_port

    def world_size(self):
        return the_world_size


trainer = Trainer(plugins=[MyCluster()], ...)

prepare_data_per_node

If True will call prepare_data() on LOCAL_RANK=0 for every node. If False will only call from NODE_RANK=0, LOCAL_RANK=0

.. testcode::

    # default
    Trainer(prepare_data_per_node=True)

    # use only NODE_RANK=0, LOCAL_RANK=0
    Trainer(prepare_data_per_node=False)

precision

Lightning supports either double (64), float (32), bfloat16 (bf16), or half (16) precision training.

Half precision, or mixed precision, is the combined use of 32 and 16 bit floating points to reduce memory footprint during model training. This can result in improved performance, achieving +3X speedups on modern GPUs.

.. testcode::
    :skipif: not torch.cuda.is_available()

    # default used by the Trainer
    trainer = Trainer(precision=32)

    # 16-bit precision
    trainer = Trainer(precision=16, gpus=1)  # works only on CUDA

    # bfloat16 precision
    trainer = Trainer(precision="bf16")

    # 64-bit precision
    trainer = Trainer(precision=64)

Note

When running on TPUs, torch.bfloat16 will be used but tensor printing will still show torch.float32.

If you are interested in using Apex 16-bit training:

NVIDIA Apex and DDP have instability problems. We recommend using the native AMP for 16-bit precision with multiple GPUs. To use Apex 16-bit training:

Install apex.

Set the precision trainer flag to 16. You can customize the Apex optimization level by setting the amp_level flag.
.. testcode::
    :skipif: not _APEX_AVAILABLE or not torch.cuda.is_available()

    # turn on 16-bit
    trainer = Trainer(amp_backend="apex", amp_level="O2", precision=16, gpus=1)

process_position

Warning

process_position has been deprecated in v1.5 and will be removed in v1.7. Please pass :class:`~pytorch_lightning.callbacks.progress.TQDMProgressBar` with process_position directly to the Trainer's callbacks argument instead.

Orders the progress bar. Useful when running multiple trainers on the same node.

.. testcode::

    # default used by the Trainer
    trainer = Trainer(process_position=0)

Note

This argument is ignored if a custom callback is passed to :paramref:`~Trainer.callbacks`.

profiler

To profile individual steps during training and assist in identifying bottlenecks.

See the :doc:`profiler documentation <../advanced/profiler>`. for more details.

.. testcode::

    from pytorch_lightning.profiler import SimpleProfiler, AdvancedProfiler

    # default used by the Trainer
    trainer = Trainer(profiler=None)

    # to profile standard training events, equivalent to `profiler=SimpleProfiler()`
    trainer = Trainer(profiler="simple")

    # advanced profiler for function-level stats, equivalent to `profiler=AdvancedProfiler()`
    trainer = Trainer(profiler="advanced")

progress_bar_refresh_rate

Warning

progress_bar_refresh_rate has been deprecated in v1.5 and will be removed in v1.7. Please pass :class:`~pytorch_lightning.callbacks.progress.TQDMProgressBar` with refresh_rate directly to the Trainer's callbacks argument instead. To disable the progress bar, pass enable_progress_bar = False to the Trainer.

How often to refresh progress bar (in steps).

.. testcode::

    # default used by the Trainer
    trainer = Trainer(progress_bar_refresh_rate=1)

    # disable progress bar
    trainer = Trainer(progress_bar_refresh_rate=0)

Note:

In Google Colab notebooks, faster refresh rates (lower number) is known to crash them because of their screen refresh rates. Lightning will set it to 20 in these environments if the user does not provide a value.
This argument is ignored if a custom callback is passed to :paramref:`~Trainer.callbacks`.

enable_progress_bar

Whether to enable or disable the progress bar. Defaults to True.

.. testcode::

    # default used by the Trainer
    trainer = Trainer(enable_progress_bar=True)

    # disable progress bar
    trainer = Trainer(enable_progress_bar=False)

reload_dataloaders_every_n_epochs

Set to a postive integer to reload dataloaders every n epochs.

# if 0 (default)
train_loader = model.train_dataloader()
for epoch in epochs:
    for batch in train_loader:
        ...

# if a positive integer
for epoch in epochs:
    if not epoch % reload_dataloaders_every_n_epochs:
        train_loader = model.train_dataloader()
    for batch in train_loader:
        ...

replace_sampler_ddp

Enables auto adding of :class:`~torch.utils.data.distributed.DistributedSampler`. In PyTorch, you must use it in distributed settings such as TPUs or multi-node. The sampler makes sure each GPU sees the appropriate part of your data. By default it will add shuffle=True for train sampler and shuffle=False for val/test sampler. If you want to customize it, you can set replace_sampler_ddp=False and add your own distributed sampler. If replace_sampler_ddp=True and a distributed sampler was already added, Lightning will not replace the existing one.

.. testcode::

    # default used by the Trainer
    trainer = Trainer(replace_sampler_ddp=True)

By setting to False, you have to add your own distributed sampler:

# in your LightningModule or LightningDataModule
def train_dataloader(self):
    # default used by the Trainer
    sampler = torch.utils.data.distributed.DistributedSampler(dataset, shuffle=True)
    dataloader = DataLoader(dataset, batch_size=32, sampler=sampler)
    return dataloader

Note

For iterable datasets, we don't do this automatically.

resume_from_checkpoint

Warning

resume_from_checkpoint is deprecated in v1.5 and will be removed in v1.7. Please pass trainer.fit(ckpt_path="some/path/to/my_checkpoint.ckpt") instead.

To resume training from a specific checkpoint pass in the path here. If resuming from a mid-epoch checkpoint, training will start from the beginning of the next epoch.

.. testcode::

    # default used by the Trainer
    trainer = Trainer(resume_from_checkpoint=None)

    # resume from a specific checkpoint
    trainer = Trainer(resume_from_checkpoint="some/path/to/my_checkpoint.ckpt")

strategy

Supports passing different training strategies with aliases (ddp, ddp_spawn, etc) as well as custom training type plugins.

# Training with the DistributedDataParallel strategy on 4 gpus
trainer = Trainer(strategy="ddp", accelerator="gpu", devices=4)

# Training with the DDP Spawn strategy using 4 cpu processes
trainer = Trainer(strategy="ddp_spawn", accelerator="cpu", devices=4)

Note

Additionally, you can pass your custom training type plugins to the strategy argument.

from pytorch_lightning.plugins import DDPPlugin


class CustomDDPPlugin(DDPPlugin):
    def configure_ddp(self):
        self._model = MyCustomDistributedDataParallel(
            self.model,
            device_ids=...,
        )


trainer = Trainer(strategy=CustomDDPPlugin(), accelerator="gpu", devices=2)

See Also:

:doc:`Multi-GPU training guide <../advanced/multi_gpu>`.
:doc:`Model Parallel GPU training guide <../advanced/advanced_gpu>`.
:doc:`TPU training guide <../advanced/tpu>`.

sync_batchnorm

Enable synchronization between batchnorm layers across all GPUs.

.. testcode::

    trainer = Trainer(sync_batchnorm=True)

track_grad_norm

no tracking (-1)
Otherwise tracks that norm (2 for 2-norm)

.. testcode::

    # default used by the Trainer
    trainer = Trainer(track_grad_norm=-1)

    # track the 2-norm
    trainer = Trainer(track_grad_norm=2)

tpu_cores

How many TPU cores to train on (1 or 8).
Which TPU core to train on [1-8]

A single TPU v2 or v3 has 8 cores. A TPU pod has up to 2048 cores. A slice of a POD means you get as many cores as you request.

Your effective batch size is batch_size * total tpu cores.

This parameter can be either 1 or 8.

Example:

# your_trainer_file.py

# default used by the Trainer (ie: train on CPU)
trainer = Trainer(tpu_cores=None)

# int: train on a single core
trainer = Trainer(tpu_cores=1)

# list: train on a single selected core
trainer = Trainer(tpu_cores=[2])

# int: train on all cores few cores
trainer = Trainer(tpu_cores=8)

# for 8+ cores must submit via xla script with
# a max of 8 cores specified. The XLA script
# will duplicate script onto each TPU in the POD
trainer = Trainer(tpu_cores=8)

To train on more than 8 cores (ie: a POD), submit this script using the xla_dist script.

Example:

python -m torch_xla.distributed.xla_dist
--tpu=$TPU_POD_NAME
--conda-env=torch-xla-nightly
--env=XLA_USE_BF16=1
-- python your_trainer_file.py

val_check_interval

How often within one training epoch to check the validation set. Can specify as float or int.

use (float) to check within a training epoch
use (int) to check every n steps (batches)

.. testcode::

    # default used by the Trainer
    trainer = Trainer(val_check_interval=1.0)

    # check validation set 4 times during a training epoch
    trainer = Trainer(val_check_interval=0.25)

    # check validation set every 1000 training batches
    # use this when using iterableDataset and your dataset has no length
    # (ie: production cases with streaming data)
    trainer = Trainer(val_check_interval=1000)

# Here is the computation to estimate the total number of batches seen within an epoch.

# Find the total number of train batches
total_train_batches = total_train_samples // (train_batch_size * world_size)

# Compute how many times we will call validation during the training loop
val_check_batch = max(1, int(total_train_batches * val_check_interval))
val_checks_per_epoch = total_train_batches / val_check_batch

# Find the total number of validation batches
total_val_batches = total_val_samples // (val_batch_size * world_size)

# Total number of batches run
total_fit_batches = total_train_batches + total_val_batches

weights_save_path

Directory of where to save weights if specified.

.. testcode::

    # default used by the Trainer
    trainer = Trainer(weights_save_path=os.getcwd())

    # save to your custom path
    trainer = Trainer(weights_save_path="my/path")

Example:

# if checkpoint callback used, then overrides the weights path
# **NOTE: this saves weights to some/path NOT my/path
checkpoint = ModelCheckpoint(dirpath='some/path')
trainer = Trainer(
    callbacks=[checkpoint],
    weights_save_path='my/path'
)

weights_summary

Warning

weights_summary is deprecated in v1.5 and will be removed in v1.7. Please pass :class:`~pytorch_lightning.callbacks.model_summary.ModelSummary` directly to the Trainer's callbacks argument instead. To disable the model summary, pass enable_model_summary = False to the Trainer.

Prints a summary of the weights when training begins. Options: 'full', 'top', None.

.. testcode::

    # default used by the Trainer (ie: print summary of top level modules)
    trainer = Trainer(weights_summary="top")

    # print full summary of all modules and submodules
    trainer = Trainer(weights_summary="full")

    # don't print a summary
    trainer = Trainer(weights_summary=None)

enable_model_summary

Whether to enable or disable the model summarization. Defaults to True.

.. testcode::

    # default used by the Trainer
    trainer = Trainer(enable_model_summary=True)

    # disable summarization
    trainer = Trainer(enable_model_summary=False)

    # enable custom summarization
    from pytorch_lightning.callbacks import ModelSummary

    trainer = Trainer(enable_model_summary=True, callbacks=[ModelSummary(max_depth=-1)])

Trainer class API

Methods

init

.. automethod:: pytorch_lightning.trainer.Trainer.__init__
   :noindex:

fit

.. automethod:: pytorch_lightning.trainer.Trainer.fit
   :noindex:

validate

.. automethod:: pytorch_lightning.trainer.Trainer.validate
   :noindex:

test

.. automethod:: pytorch_lightning.trainer.Trainer.test
   :noindex:

predict

.. automethod:: pytorch_lightning.trainer.Trainer.predict
   :noindex:

tune

.. automethod:: pytorch_lightning.trainer.Trainer.tune
   :noindex:

Properties

callback_metrics

The metrics available to callbacks. These are automatically set when you log via self.log

def training_step(self, batch, batch_idx):
    self.log("a_val", 2)


callback_metrics = trainer.callback_metrics
assert callback_metrics["a_val"] == 2

current_epoch

The current epoch

def training_step(self, batch, batch_idx):
    current_epoch = self.trainer.current_epoch
    if current_epoch > 100:
        # do something
        pass

logger (p)

The current logger being used. Here's an example using tensorboard

def training_step(self, batch, batch_idx):
    logger = self.trainer.logger
    tensorboard = logger.experiment

logged_metrics

The metrics sent to the logger (visualizer).

def training_step(self, batch, batch_idx):
    self.log("a_val", 2, logger=True)


logged_metrics = trainer.logged_metrics
assert logged_metrics["a_val"] == 2

log_dir

The directory for the current experiment. Use this to save images to, etc...

def training_step(self, batch, batch_idx):
    img = ...
    save_img(img, self.trainer.log_dir)

is_global_zero

Whether this process is the global zero in multi-node training

def training_step(self, batch, batch_idx):
    if self.trainer.is_global_zero:
        print("in node 0, accelerator 0")

progress_bar_metrics

The metrics sent to the progress bar.

def training_step(self, batch, batch_idx):
    self.log("a_val", 2, prog_bar=True)


progress_bar_metrics = trainer.progress_bar_metrics
assert progress_bar_metrics["a_val"] == 2

Files

trainer.rst

Latest commit

History

trainer.rst

File metadata and controls

Trainer

Basic use

Under the hood

Trainer in Python scripts

Validation

Testing

Reproducibility

Trainer flags

accelerator

accumulate_grad_batches

amp_backend

amp_level

auto_scale_batch_size

auto_select_gpus

auto_lr_find

benchmark

deterministic

callbacks

check_val_every_n_epoch

checkpoint_callback

default_root_dir

devices

enable_checkpointing

fast_dev_run

flush_logs_every_n_steps

gpus

gradient_clip_val

limit_train_batches

limit_test_batches

limit_val_batches

log_every_n_steps

logger

max_epochs

min_epochs

max_steps

min_steps

max_time

num_nodes

num_processes

num_sanity_val_steps

overfit_batches

plugins

prepare_data_per_node

precision

process_position

profiler

progress_bar_refresh_rate

enable_progress_bar

reload_dataloaders_every_n_epochs

replace_sampler_ddp

resume_from_checkpoint

strategy

sync_batchnorm

track_grad_norm

tpu_cores

val_check_interval

weights_save_path

weights_summary

enable_model_summary

Trainer class API

Methods

init

fit

validate

test

predict

tune

Properties

callback_metrics

current_epoch

logger (p)

logged_metrics

log_dir

is_global_zero