docs/source/data.rst

torch.utils.data
===================================

.. automodule:: torch.utils.data

At the heart of PyTorch data loading utility is the :class:`torch.utils.data.DataLoader`
class.  It represents a Python iterable over a dataset, with support for

* `map-style and iterable-style datasets <Dataset Types_>`_,

* `customizing data loading order <Data Loading Order and Sampler_>`_,

* `automatic batching <Loading Batched and Non-Batched Data_>`_,

* `single- and multi-process data loading <Single- and Multi-process Data Loading_>`_,

* `automatic memory pinning <Memory Pinning_>`_.

These options are configured by the constructor arguments of a
:class:`~torch.utils.data.DataLoader`, which has signature::

    DataLoader(dataset, batch_size=1, shuffle=False, sampler=None,
               batch_sampler=None, num_workers=0, collate_fn=None,
               pin_memory=False, drop_last=False, timeout=0,
               worker_init_fn=None)

The sections below describe in details the effects and usages of these options.

Dataset Types
-------------

The most important argument of :class:`~torch.utils.data.DataLoader`
constructor is :attr:`dataset`, which indicates a dataset object to load data
from. PyTorch supports two different types of datasets:

* `map-style datasets <Map-style datasets_>`_,

* `iterable-style datasets <Iterable-style datasets_>`_.

Map-style datasets
^^^^^^^^^^^^^^^^^^

A map-style dataset is one that implements the :meth:`__getitem__` and
:meth:`__len__` protocols, and represents a map from (possibly non-integral)
indices/keys to data samples.

For example, such a dataset, when accessed with ``dataset[idx]``, could read
the ``idx``-th image and its corresponding label from a folder on the disk.

See :class:`~torch.utils.data.Dataset` for more details.

Iterable-style datasets
^^^^^^^^^^^^^^^^^^^^^^^

An iterable-style dataset is an instance of a subclass of :class:`~torch.utils.data.IterableDataset`
that implements the :meth:`__iter__` protocol, and represents an iterable over
data samples. This type of datasets is particularly suitable for cases where
random reads are expensive or even improbable, and where the batch size depends
on the fetched data.

For example, such a dataset, when called ``iter(dataset)``, could return a
stream of data reading from a database, a remote server, or even logs generated
in real time.

See :class:`~torch.utils.data.IterableDataset` for more details.

.. note:: When using an :class:`~torch.utils.data.IterableDataset` with
          `multi-process data loading <Multi-process data loading_>`_. The same
          dataset object is replicated on each worker process, and thus the
          replicas must be configured differently to avoid duplicated data. See
          :class:`~torch.utils.data.IterableDataset` documentations for how to
          achieve this.

Data Loading Order and :class:`~torch.utils.data.Sampler`
---------------------------------------------------------

For `iterable-style datasets <Iterable-style datasets_>`_, data loading order
is entirely controlled by the user-defined iterable. This allows easier
implementations of chunk-reading and dynamic batch size (e.g., by yielding a
batched sample at each time).

The rest of this section concerns the case with
`map-style datasets <Map-style datasets_>`_. :class:`torch.utils.data.Sampler`
classes are used to specify the sequence of indices/keys used in data loading.
They represent iterable objects over the indices to datasets.  E.g., in the
common case with stochastic gradient decent (SGD), a
:class:`~torch.utils.data.Sampler` could randomly permute a list of indices
and yield each one at a time, or yield a small number of them for mini-batch
SGD.

A sequential or shuffled sampler will be automatically constructed based on the :attr:`shuffle` argument to a :class:`~torch.utils.data.DataLoader`.
Alternatively, users may use the :attr:`sampler` argument to specify a
custom :class:`~torch.utils.data.Sampler` object that at each time yields
the next index/key to fetch.

A custom :class:`~torch.utils.data.Sampler` that yields a list of batch
indices at a time can be passed as the :attr:`batch_sampler` argument.
Automatic batching can also be enabled via :attr:`batch_size` and
:attr:`drop_last` arguments. See
`the next section <Loading Batched and Non-Batched Data_>`_ for more details
on this.

.. note::
  Neither :attr:`sampler` nor :attr:`batch_sampler` is compatible with
  iterable-style datasets, since such datasets have no notion of a key or an
  index.

Loading Batched and Non-Batched Data
------------------------------------

:class:`~torch.utils.data.DataLoader` supports automatically collating
individual fetched data samples into batches via arguments
:attr:`batch_size`, :attr:`drop_last`, and :attr:`batch_sampler`.


Automatic batching (default)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This is the most common case, and corresponds to fetching a minibatch of
data and collating them into batched samples, i.e., containing Tensors with
one dimension being the batch dimension (usually the first).

When :attr:`batch_size` (default ``1``) is not ``None``, the data loader yields
batched samples instead of individual samples. :attr:`batch_size` and
:attr:`drop_last` arguments are used to specify how the data loader obtains
batches of dataset keys. For map-style datasets, users can alternatively
specify :attr:`batch_sampler`, which yields a list of keys at a time.

.. note::
  The :attr:`batch_size` and :attr:`drop_last` arguments essentially are used
  to construct a :attr:`batch_sampler` from :attr:`sampler`. For map-style
  datasets, the :attr:`sampler` is either provided by user or constructed
  based on the :attr:`shuffle` argument. For iterable-style datasets, the
  :attr:`sampler` is a dummy infinite one. See
  `this section <Data Loading Order and Sampler_>`_ on more details on
  samplers.

.. note::
  When fetching from
  `iterable-style datasets <Iterable-style datasets_>`_ with
  `multi-processing <Multi-process data loading_>`_, the :attr:`drop_last`
  argument drops the last non-full batch of each worker's dataset replica.

After fetching a list of samples using the indices from sampler, the function
passed as the :attr:`collate_fn` argument is used to collate lists of samples
into batches.

In this case, loading from a map-style dataset is roughly equivalent with::

    for indices in batch_sampler:
        yield collate_fn([dataset[i] for i in indices])

and loading from an iterable-style dataset is roughly equivalent with::

    dataset_iter = iter(dataset)
    for indices in batch_sampler:
        yield collate_fn([next(dataset_iter) for _ in indices])

A custom :attr:`collate_fn` can be used to customize collation, e.g., padding
sequential data to max length of a batch. See
`this section <dataloader-collate_fn_>`_ on more about :attr:`collate_fn`.

Disable automatic batching
^^^^^^^^^^^^^^^^^^^^^^^^^^

In certain cases, users may want to handle batching manually in dataset code,
or simply load individual samples. For example, it could cheaper to directly
load batched data (e.g., bulk reads from a database or reading continuous
chunks of memory), or the batch size is data dependent, or the program is
designed to work on individual samples.  Under these scenarios, it's likely
better to not use automatic batching (where :attr:`collate_fn` is used to
collate the samples), but let the data loader directly return each member of
the :attr:`dataset` object.

When both :attr:`batch_size` and :attr:`batch_sampler` are ``None``, automatic
batching is disabled. Each sample obtained from the :attr:`dataset` is
processed with the function passed as the :attr:`collate_fn` argument.

**When automatic batching is disabled**, the default :attr:`collate_fn` simply
converts NumPy arrays into PyTorch Tensors, and keeps everything else untouched.

In this case, loading from a map-style dataset is roughly equivalent with::

    for index in sampler:
        yield collate_fn(dataset[index])

and loading from an iterable-style dataset is roughly equivalent with::

    for data in iter(dataset):
        yield collate_fn(data)

See `this section <dataloader-collate_fn_>`_ on more about :attr:`collate_fn`.

.. _dataloader-collate_fn:

Working with :attr:`collate_fn`
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The use of :attr:`collate_fn` is slightly different when automatic batching is
enabled or disabled.

**When automatic batching is disabled**, :attr:`collate_fn` is called with
each individual data sample, and the output is yielded from the data loader
iterator. In this case, the default :attr:`collate_fn` simply converts NumPy
arrays in PyTorch tensors.

**When automatic batching is enabled**, :attr:`collate_fn` is called with a list
of data samples at each time. It is expected to collate the input samples into
a batch for yielding from the data loader iterator. The rest of this section
describes behavior of the default :attr:`collate_fn` in this case.

For instance, if each data sample consists of a 3-channel image and an integral
class label, i.e., each element of the dataset returns a tuple
``(image, class_index)``, the default :attr:`collate_fn` collates a list of
such tuples into a single tuple of a batched image tensor and a batched class
label Tensor. In particular, the default :attr:`collate_fn` has the following
properties:

* It always prepends a new dimension as the batch dimension.

* It automatically converts NumPy arrays and Python numerical values into
  PyTorch Tensors.

* It preserves the data structure, e.g., if each sample is a dictionary, it
  outputs a dictionary with the same set of keys but batched Tensors as values
  (or lists if the values can not be converted into Tensors). Same
  for ``list`` s, ``tuple`` s, ``namedtuple`` s, etc.

Users may use customized :attr:`collate_fn` to achieve custom batching, e.g.,
collating along a dimension other than the first, padding sequences of
various lengths, or adding support for custom data types.

Single- and Multi-process Data Loading
--------------------------------------

A :class:`~torch.utils.data.DataLoader` uses single-process data loading by
default.

Within a Python process, the
`Global Interpreter Lock (GIL) <https://wiki.python.org/moin/GlobalInterpreterLock>`_
prevents true fully parallelizing Python code across threads. To avoid blocking
computation code with data loading, PyTorch provides an easy switch to perform
multi-process data loading by simply setting the argument :attr:`num_workers`
to a positive integer.

Single-process data loading (default)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In this mode, data fetching is done in the same process a
:class:`~torch.utils.data.DataLoader` is initialized.  Therefore, data loading
may block computing.  However, this mode may be preferred when resource(s) used
for sharing data among processes (e.g., shared memory, file descriptors) is
limited, or when the entire dataset is small and can be loaded entirely in
memory.  Additionally, single-process loading often shows more readable error
traces and thus is useful for debugging.


Multi-process data loading
^^^^^^^^^^^^^^^^^^^^^^^^^^

Setting the argument :attr:`num_workers` as a positive integer will
turn on multi-process data loading with the specified number of loader worker
processes.

In this mode, each time an iterator of a :class:`~torch.utils.data.DataLoader`
is created (e.g., when you call ``enumerate(dataloader)``), :attr:`num_workers`
worker processes are created. At this point, the :attr:`dataset`,
:attr:`collate_fn`, and :attr:`worker_init_fn` are passed to each
worker, where they are used to initialize, and fetch data. This means that
dataset access together with its  internal IO, transforms
(including :attr:`collate_fn`) runs in the worker process.

:func:`torch.utils.data.get_worker_info()` returns various useful information
in a worker process (including the worker id, dataset replica, initial seed,
etc.), and returns ``None`` in main process. Users may use this function in
dataset code and/or :attr:`worker_init_fn` to individually configure each
dataset replica, and to determine whether the code is running in a worker
process. For example, this can be particularly helpful in sharding the dataset.

For map-style datasets, the main process generates the indices using
:attr:`sampler` and sends them to the workers. So any shuffle randomization is
done in the main process which guides loading by assigning indices to load.

For iterable-style datasets, since each worker process gets a replica of the
:attr:`dataset` object, naive multi-process loading will often result in
duplicated data. Using :func:`torch.utils.data.get_worker_info()` and/or
:attr:`worker_init_fn`, users may configure each replica independently. (See
:class:`~torch.utils.data.IterableDataset` documentations for how to achieve
this. ) For similar reasons, in multi-process loading, the :attr:`drop_last`
argument drops the last non-full batch of each worker's iterable-style dataset
replica.

Workers are shut down once the end of the iteration is reached, or when the
iterator becomes garbage collected.

.. warning::
  It is generally not recommended to return CUDA tensors in multi-process
  loading because of many subtleties in using CUDA and sharing CUDA tensors in
  multiprocessing (see :ref:`multiprocessing-cuda-note`). Instead, we recommend
  using `automatic memory pinning <Memory Pinning_>`_ (i.e., setting
  :attr:`pin_memory=True`), which enables fast data transfer to CUDA-enabled
  GPUs.

Platform-specific behaviors
"""""""""""""""""""""""""""

Since workers rely on Python :py:mod:`multiprocessing`, worker launch behavior is
different on Windows compared to Unix.

* On Unix, :func:`fork()` is the default :py:mod:`multiprocessing` start method.
  Using :func:`fork`, child workers typically can access the :attr:`dataset` and
  Python argument functions directly through the cloned address space.

* On Windows, :func:`spawn()` is the default :py:mod:`multiprocessing` start method.
  Using :func:`spawn()`, another interpreter is launched which runs your main script,
  followed by the internal worker function that receives the :attr:`dataset`,
  :attr:`collate_fn` and other arguments through :py:mod:`pickle` serialization.

This separate serialization means that you should take two steps to ensure you
are compatible with Windows while using multi-process data loading:

- Wrap most of you main script's code within ``if __name__ == '__main__':`` block,
  to make sure it doesn't run again (most likely generating error) when each worker
  process is launched. You can place your dataset and :class:`~torch.utils.data.DataLoader`
  instance creation logic here, as it doesn't need to be re-executed in workers.

- Make sure that any custom :attr:`collate_fn`, :attr:`worker_init_fn`
  or :attr:`dataset` code is declared as top level definitions, outside of the
  ``__main__`` check. This ensures that they are available in worker processes.
  (this is needed since functions are pickled as references only, not ``bytecode``.)

Randomness in multi-process data loading
""""""""""""""""""""""""""""""""""""""""""

By default, each worker will have its PyTorch seed set to ``base_seed + worker_id``,
where ``base_seed`` is a long generated by main process using its RNG (thereby, 
consuming a RNG state mandatorily). However, seeds for other libraries may be 
duplicated upon initializing workers (w.g., NumPy), causing each worker to return 
identical random numbers. (See :ref:`this section <dataloader-workers-random-seed>` in FAQ.).

In :attr:`worker_init_fn`, you may access the PyTorch seed set for each worker
with either :func:`torch.utils.data.get_worker_info().seed <torch.utils.data.get_worker_info>`
or :func:`torch.initial_seed()`, and use it to seed other libraries before data
loading.

Memory Pinning
--------------

Host to GPU copies are much faster when they originate from pinned (page-locked)
memory. See :ref:`cuda-memory-pinning` for more details on when and how to use
pinned memory generally.

For data loading, passing :attr:`pin_memory=True` to a
:class:`~torch.utils.data.DataLoader` will automatically put the fetched data
Tensors in pinned memory, and thus enables faster data transfer to CUDA-enabled
GPUs.

The default memory pinning logic only recognizes Tensors and maps and iterables
containing Tensors.  By default, if the pinning logic sees a batch that is a
custom type (which will occur if you have a :attr:`collate_fn` that returns a
custom batch type), or if each element of your batch is a custom type, the
pinning logic will not recognize them, and it will return that batch (or those
elements) without pinning the memory.  To enable memory pinning for custom
batch or data type(s), define a :meth:`pin_memory` method on your custom
type(s).

See the example below.

Example::

    class SimpleCustomBatch:
        def __init__(self, data):
            transposed_data = list(zip(*data))
            self.inp = torch.stack(transposed_data[0], 0)
            self.tgt = torch.stack(transposed_data[1], 0)

        # custom memory pinning method on custom type
        def pin_memory(self):
            self.inp = self.inp.pin_memory()
            self.tgt = self.tgt.pin_memory()
            return self

    def collate_wrapper(batch):
        return SimpleCustomBatch(batch)

    inps = torch.arange(10 * 5, dtype=torch.float32).view(10, 5)
    tgts = torch.arange(10 * 5, dtype=torch.float32).view(10, 5)
    dataset = TensorDataset(inps, tgts)

    loader = DataLoader(dataset, batch_size=2, collate_fn=collate_wrapper,
                        pin_memory=True)

    for batch_ndx, sample in enumerate(loader):
        print(sample.inp.is_pinned())
        print(sample.tgt.is_pinned())


.. autoclass:: DataLoader
.. autoclass:: Dataset
.. autoclass:: IterableDataset
.. autoclass:: TensorDataset
.. autoclass:: ConcatDataset
.. autoclass:: ChainDataset
.. autoclass:: Subset
.. autofunction:: torch.utils.data.get_worker_info
.. autofunction:: torch.utils.data.random_split
.. autoclass:: torch.utils.data.Sampler
.. autoclass:: torch.utils.data.SequentialSampler
.. autoclass:: torch.utils.data.RandomSampler
.. autoclass:: torch.utils.data.SubsetRandomSampler
.. autoclass:: torch.utils.data.WeightedRandomSampler
.. autoclass:: torch.utils.data.BatchSampler
.. autoclass:: torch.utils.data.distributed.DistributedSampler