PyTorch API

In this guide, you’ll learn how to use the PyTorch API.

Visit the API reference

PyTorch API Reference

This document guides you through training a PyTorch model in Determined. You need to implement a trial class that inherits from PyTorchTrial and specify it as the entrypoint in the experiment configuration.

To implement a PyTorchTrial, you need to override specific functions that represent the components that are used in the training procedure. It is helpful to work off of a skeleton to keep track of what is still required. A good starting template can be found below:

from typing import Any, Dict, Union, Sequence
from determined.pytorch import DataLoader, PyTorchTrial, PyTorchTrialContext

TorchData = Union[Dict[str, torch.Tensor], Sequence[torch.Tensor], torch.Tensor]

class MyTrial(PyTorchTrial):
    def __init__(self, context: PyTorchTrialContext) -> None:
        self.context = context

    def build_training_data_loader(self) -> DataLoader:
        return DataLoader()

    def build_validation_data_loader(self) -> DataLoader:
        return DataLoader()

    def train_batch(self, batch: TorchData, epoch_idx: int, batch_idx: int)  -> Dict[str, Any]:
        return {}

    def evaluate_batch(self, batch: TorchData) -> Dict[str, Any]:
        return {}

To learn more about the PyTorch API, you can start by reading the trial definitions from the following examples:

For tips on debugging, see How to Debug Models.

Download Data


Before continuing, read how to Prepare Data to understand how to work with different sources of data.

There are two ways to download your dataset in the PyTorch API:

  1. Download the data in the

  2. Download the data in the constructor function __init__() of PyTorchTrial.

If you are running a distributed training experiment, we suggest you to use the second approach. During distributed training, a trial needs running multiple processes on different containers. In order for all the processes to have access to the data and to prevent multiple download download processes (one process per GPU) from conflicting with one another, the data should be downloaded to unique directories for different ranks.

See the following code as an example:

def __init__(self, context) -> None:
    self.context = context

    # Create a unique download directory for each rank so they don't overwrite each
    # other when doing distributed training.
    self.download_directory = f"/tmp/data-rank{self.context.distributed.get_rank()}"
    self.download_directory = download_data(

Load Data

Loading data into PyTorchTrial models is done by defining two functions, build_training_data_loader() and build_validation_data_loader(). Each function should return an instance of determined.pytorch.DataLoader.

The determined.pytorch.DataLoader class behaves the same as and is a drop-in replacement in most cases. It handles distributed training with PyTorchTrial.

Each determined.pytorch.DataLoader will return batches of data, which will be fed directly to the train_batch() and evaluate_batch() functions. The batch size of the data loader will be set to the per-slot batch size, which is calculated based on global_batch_size and slots_per_trial as defined in the experiment configuration.

See the following code as an example:

def build_training_data_loader(self):
    traindir = os.path.join(self.download_directory, 'train')
    self.normalize = transforms.Normalize(mean=[0.485, 0.456, 0.406],
                                std=[0.229, 0.224, 0.225])

    train_dataset = datasets.ImageFolder(

    train_loader = determined.pytorch.DataLoader(
        num_workers=self.context.get_hparam("workers", pin_memory=True),
    return train_loader

The output train_batch() returns a batch of data in one of the following formats:

# A numpy array
batch: np.ndarray = np.array([0, 0], [0, 0]])
# A PyTorch tensor
batch: torch.Tensor = torch.Tensor([[0, 0], [0, 0]])
# A tuple of arrays or tensors
batch: Tuple[np.ndarray] = (np.array([0, 0]), np.array([0, 0]))
batch: Tuple[torch.Tensor] = (torch.Tensor([0, 0]), torch.Tensor([0, 0]))
# A list of arrays or tensors
batch: List[np.ndarray] = [np.array([0, 0]), np.array([0, 0])]
batch: List[torch.Tensor] = [torch.Tensor([0, 0]), torch.Tensor([0, 0])]
# A dictionary mapping strings to arrays or tensors
batch: Dict[str, np.ndarray] = {"data": np.array([0, 0]), "label": np.array([0, 0])}
batch: Dict[str, torch.Tensor] = {"data": torch.Tensor([0, 0]), "label": torch.Tensor([0, 0])}
# A combination of the above
batch = {
    "data": [
        {"sub_data1": torch.Tensor([[0, 0], [0, 0]])},
        {"sub_data2": torch.Tensor([0, 0])},
    "label": (torch.Tensor([0, 0]), torch.Tensor([[0, 0], [0, 0]])),

Initializing Objects

You need to initialize the objects that will be used in training in the constructor __init__() of determined.pytorch.PyTorchTrial using the provided context: these objects include the model(s), optimizer(s), learning rate scheduler(s), and custom loss and metric functions. See __init__() for details.


Be sure to wrap your objects! You may see metrics for trials that are paused and later continued that are significantly different from trials that are not paused if some of your models, optimizers, and learning rate schedulers are not wrapped. The reason is that the model’s state may not be restored accurately or completely from the checkpoint, which is saved to a checkpoint and then later loaded into the trial during resumed training. When using PyTorch, this can sometimes happen if the PyTorch API is not used correctly.


You need to call the wrap_optimizer() method of the PyTorchTrialContext to wrap your instantiated optimizers in the __init__() constructor. For example,

def __init__(self, context: PyTorchTrialContext):
    self.context = context

    optimizer = torch.optim.SGD(
    self.optimizer = self.context.wrap_optimizer(optimizer)

Then you need to step your optimizer in the train_batch() (see Optimization Step below).

Learning Rate Schedulers

Determined has a few ways of managing the learning rate. Determined can automatically update every batch or epoch, or you can manage it yourself.

You need to call the wrap_lr_scheduler() method of the PyTorchTrialContext to wrap your instantiated learning rate schedulers in the __init__() constructor. For example,

def __init__(self, context: PyTorchTrialContext):
    self.context = context

    lr_sch = torch.optim.lr_scheduler.StepLR(self.optimizer, gamma=.1, step_size=2)
    self.lr_sch = self.context.wrap_lr_scheduler(

If your learning rate scheduler uses the manual step mode, you will need to step your learning rate scheduler in the train_batch() method of PyTorchTrial by calling:

def train_batch(self, batch: pytorch.TorchData, epoch_idx: int, batch_idx: int)

Define the Training Loop

Optimization Step

You need to implement the train_batch() method of your PyTorchTrial subclass.

Typically when training with native PyTorch, you write a training loop, which iterates through the dataloader to access and train your model one batch at a time. You can usually identify this code by finding the common code snippet: for batch in dataloader. In Determined, train_batch() also works with one batch at a time.

Take this script implemented with the native PyTorch as an example. It has the following code for the training loop.

for i, (images, target) in enumerate(train_loader):
    # measure data loading time
    data_time.update(time.time() - end)

    # move data to the same device as model
    images =, non_blocking=True)
    target =, non_blocking=True)

    # compute output
    output = model(images)
    loss = criterion(output, target)

    # measure accuracy and record loss
    acc1, acc5 = accuracy(output, target, topk=(1, 5))
    losses.update(loss.item(), images.size(0))
    top1.update(acc1[0], images.size(0))
    top5.update(acc5[0], images.size(0))

    # compute gradient and do SGD step

    # measure elapsed time
    batch_time.update(time.time() - end)
    end = time.time()

    if i % args.print_freq == 0:
        progress.display(i + 1)

Notice that this pure-PyTorch loop manages the per-batch metrics. With Determined, metrics returned by train_batch() are automatically averaged and displayed, so we do not need to do this ourselves.

Next, we will convert some PyTorch functions to use Determined’s equivalents. We need to change optimizer.zero_grad(), loss.backward(), and optimizer.step(). The self.context object will be used to call loss.backwards and handle zeroing and stepping the optimizer.

The final train_batch() will look like:

def train_batch(self, batch: TorchData, epoch_idx: int, batch_idx: int):
    images, target = batch
    output = self.model(images)
    loss = self.criterion(output, target)
    acc1, acc5 = self.accuracy(output, target, topk=(1, 5))


    return {"loss": loss.item(), "top1": acc1[0], "top5": acc5[0]}


A checkpoint includes the model definition (Python source code), experiment configuration file, network architecture, and the values of the model’s parameters (i.e., weights) and hyperparameters. When using a stateful optimizer during training, checkpoints will also include the state of the optimizer (i.e., learning rate). Users can also embed arbitrary metadata in checkpoints via a Python SDK.

PyTorch trials are checkpointed as a state-dict.pth file. This file is created in a similar manner to the procedure described in the PyTorch documentation, but instead of the fields in that documentation, the dictionary will have four keys: models_state_dict, optimizers_state_dict, lr_schedulers_state_dict, and callbacks, which are the state_dict of the models, optimizers, LR schedulers, and callbacks respectively.

Define the Validation Loop

You need to implement either the evaluate_batch() or evaluate_full_dataset() method. To load data into the validation loop, define build_validation_data_loader(). To define reducing metrics, define evaluation_reducer().

For example,

def evaluate_batch(self, batch: TorchData):
    images, target = batch
    output = self.model(images)
    validation_loss = self.criterion(output, target)
    return {"validation_loss": loss.item()}


To execute arbitrary Python code during the lifecycle of a PyTorchTrial, implement the PyTorchCallback and supply them to the PyTorchTrial by implementing build_callbacks().

Advanced Usage

Gradient Clipping

Users need to pass a gradient clipping function to step_optimizer().

Reducing Metrics

Determined supports proper reduction of arbitrary training and validation metrics, even during distributed training, by allowing users to define custom reducers. Custom reducers can be either a function or an implementation of the determined.pytorch.MetricReducer interface. See determined.pytorch.PyTorchTrialContext.wrap_reducer() for more details.

Customize a Reproducible Dataset


Normally, using determined.pytorch.DataLoader is required and handles all of the below details without any special effort on your part (see Load Data). When determined.pytorch.DataLoader is not suitable (especially in the case of IterableDatasets), you may disable this requirement by calling context.experimental.disable_dataset_reproducibility_checks() in your Trial’s __init__() method. Then you may choose to follow the below guidelines for ensuring dataset reproducibility on your own.

Achieving a reproducible dataset that is able to pause and continue (sometimes called “incremental training”) is easy if you follow a few rules.

  • Even if you are going to ultimately return an IterableDataset, it is best to use PyTorch’s Sampler class as the basis for choosing the order of records. Operations on Samplers are quick and cheap, while operations on data afterwards are expensive. For more details, see the discussion of random vs sequential access here. If you don’t have a custom sampler, start with a simple one:

  • Shuffle first: Always use a reproducible shuffle when you shuffle. Determined provides two shuffling samplers for this purpose; the ReproducibleShuffleSampler for operating on records and the ReproducibleShuffleBatchSampler for operating on batches. You should prefer to shuffle on records (use the ReproducibleShuffleSampler) whenever possible, to achieve the highest-quality shuffle.

  • Repeat when training: In Determined, you always repeat your training dataset and you never repeat your validation datasets. Determined provides a RepeatSampler and a RepeatBatchSampler to wrap your sampler or batch_sampler. For your training dataset, make sure that you always repeat AFTER you shuffle, otherwise your shuffle will hang.

  • Always shard, and not before a repeat: Use Determined’s DistributedSampler or DistributedBatchSampler to provide a unique shard of data to each worker based on your sampler or batch_sampler. It is best to always shard your data, and even when you are not doing distributed training, because in non-distributed-training settings, the sharding is nearly zero-cost, and it makes distributed training seamless if you ever want to use it in the future.

    It is generally important to shard after you repeat, unless you can guarantee that each shard of the dataset will have the same length. Otherwise, differences between the epoch boundaries for each worker can grow over time, especially on small datasets. If you shard after you repeat, you can change the number of workers arbitrarily without issue.

  • Skip when training, and always last: In Determined, training datasets should always be able to start from an arbitrary point in the dataset. This allows for advanced hyperparameter searches and responsive preemption for training on spot instances in the cloud. The easiest way to do this, which is also very efficient, is to apply a skip to the sampler.

    Determined provides a SkipBatchSampler that you can apply to your batch_sampler for this purpose. There is also a SkipSampler that you can apply to your sampler, but you should prefer to skip on batches unless you are confident that your dataset always yields identical size batches, where the number of records to skip can be reliably calculated from the number of batches already trained.

    Always skip AFTER your repeat, so that the skip only happens once, and not on every epoch.

    Always skip AFTER your shuffle, to preserve the reproducibility of the shuffle.

Here is some example code that follows each of these rules that you can use as a starting point if you find that the built-in context.DataLoader() does not support your use case.

def make_batch_sampler(
  mode,  # mode="training" or mode="validation"
    if isinstance(sampler_or_dataset,
        sampler = sampler_or_dataset
        # Create a SequentialSampler if we started with a Dataset.
        sampler =

    if mode == "training":
        # Shuffle first.
        sampler = samplers.ReproducibleShuffleSampler(sampler, shuffle_seed)

        # Repeat when training.
        sampler = samplers.RepeatSampler(sampler)

    # Always shard, and not before a repeat.
    sampler = samplers.DistributedSampler(sampler, num_workers=num_workers, rank=rank)

    # Batch before skip, because Determined counts batches, not records.
    batch_sampler =, batch_size, drop_last=False)

    if mode == "training":
        # Skip when training, and always last.
        batch_sampler = samplers.SkipBatchSampler(batch_sampler, skip)

    return batch_sampler

class MyPyTorchTrial(det.pytorch.PyTorchTrial):
    def __init__(self, context):

    def build_training_data_loader(self):
        my_dataset = ...

        batch_sampler = make_batch_sampler(

        return, batch_sampler=batch_sampler)

See the determined.pytorch.samplers for details.

Porting Checklist

If you port your code to Determined, you should walk through this checklist to ensure your code does not conflict with the Determined library.

Remove Pinned GPUs

Determined handles scheduling jobs on available slots. However, you need to let the Determined library handles choosing the GPUs.

Take this script as an example. It has the following code to configure the GPU:

if args.gpu is not None:
    print("Use GPU: {} for training".format(args.gpu))

Any use of args.gpu should be removed.

Remove Distributed Training Code

To run distributed training outside Determined, you need to have code that handles the logic of launching processes, moving models to pined GPUs, sharding data, and reducing metrics. You need to remove this code to be not conflict with the Determined library.

Take this script as an example. It has the following code to initialize the process group:

if args.distributed:
    if args.dist_url == "env://" and args.rank == -1:
        args.rank = int(os.environ["RANK"])
    if args.multiprocessing_distributed:
        # For multiprocessing distributed training, rank needs to be the
        # global rank among all the processes
        args.rank = args.rank * ngpus_per_node + gpu
    dist.init_process_group(backend=args.dist_backend, init_method=args.dist_url,
                            world_size=args.world_size, rank=args.rank)

This example also has the following code to set up CUDA and converts the model to a distributed one.

if not torch.cuda.is_available():
    print('using CPU, this will be slow')
elif args.distributed:
    # For multiprocessing distributed, DistributedDataParallel constructor
    # should always set the single device scope, otherwise,
    # DistributedDataParallel will use all available devices.
    if args.gpu is not None:
        # When using a single GPU per process and per
        # DistributedDataParallel, we need to divide the batch size
        # ourselves based on the total number of GPUs we have
        args.batch_size = int(args.batch_size / ngpus_per_node)
        args.workers = int((args.workers + ngpus_per_node - 1) / ngpus_per_node)
        model = torch.nn.parallel.DistributedDataParallel(model, device_ids=[args.gpu])
        # DistributedDataParallel will divide and allocate batch_size to all
        # available GPUs if device_ids are not set
        model = torch.nn.parallel.DistributedDataParallel(model)
elif args.gpu is not None:
    model = model.cuda(args.gpu)
    # DataParallel will divide and allocate batch_size to all available GPUs
    if args.arch.startswith('alexnet') or args.arch.startswith('vgg'):
        model.features = torch.nn.DataParallel(model.features)
        model = torch.nn.DataParallel(model).cuda()

This code is unnecessary in the trial definition. When we create the model, we will wrap it with self.context.wrap_model(model), which will convert the model to distributed if needed. We will also automatically set up horovod for you. If you would like to access the rank (typically used to view per GPU training), you can get it by calling self.context.distributed.rank.

To handle data loading in distributed training, this example has the code below:

traindir = os.path.join(, 'train')
valdir = os.path.join(, 'val')
normalize = transforms.Normalize(mean=[0.485, 0.456, 0.406],
                                std=[0.229, 0.224, 0.225])

train_dataset = datasets.ImageFolder(

# Handle distributed sampler for distributed training.
if args.distributed:
    train_sampler =
    train_sampler = None

This should be removed since we will use distributed data loader if you following the instructions of build_training_data_loader() and build_validation_data_loader().

Get Hyperparameters from PyTorchTrialContext

Take the following code for example.

def __init__(self, context: PyTorchTrialContext):
    self.context = context
    if args.pretrained:
        print("=> using pre-trained model '{}'".format(args.arch))
        model = models.__dict__[args.arch](pretrained=True)
        print("=> creating model '{}'".format(args.arch))
        model = models.__dict__[args.arch]()

args.arch is a hyperparameter. You should define the hyperparameter space in the experiment configuration and use self.context.get_hparams(), which gives you access to all the hyperparameters for the current trial. By doing so, you get better tracking in the WebUI, especially for experiments that use a searcher.