Data Loading Order and Sampler

For 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. 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 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 shuffle argument to a DataLoader. Alternatively, users may use the sampler argument to specify a custom Sampler object that at each time yields the next index/key to fetch.

A custom Sampler that yields a list of batch indices at a time can be passed as the batch_sampler argument. Automatic batching can also be enabled via batch_size and drop_last arguments. See the next section for more details on this.

NOTE

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

Sampler主要是结合Map-style的DataSet使用，所用的Sampler都有__iter__方法，返回一个取样索引的迭代器。可以选择性的实现__len__。

import torch.utils.data as data
#shape (4,1,2,2)
data_tensor=torch.Tensor(range(1,17)).reshape(4,1,2,2)


sequential_sampler=data.SequentialSampler(data_tensor) #len=4

random_sampler=data.RandomSampler(data_tensor,replacement=False, num_samples=None)#len=4


for i in random_sampler:
    print(i)
out:   
0
1
3
2


random_sampler2=data.RandomSampler(data_tensor,replacement=True, num_samples=5)#len=5,可以重复
subset_random_sampler=data.SubsetRandomSampler([0,1,3])

weighted_random_sampler1=data.WeightedRandomSampler([0.1,0.2,0.1,0.1],#共四个样本，每个样本被采到的概率
                                                   num_samples=4, #要采的样本数，<=weights中非零元素的个数
                                                   replacement=False)#不能有重复
weighted_random_sampler2=data.WeightedRandomSampler([0.1,0.2,0.1,0.1],#共四个样本，每个样本被采到的概率
                                                   num_samples=6, #要采的样本数，可以大于weights中非零元素的个数
                                                   replacement=True)#可以有重复,默认为True,不然权值就没有意义了


batch_sampler=data.BatchSampler(random_sampler,batch_size=2,drop_last=False)
list(batch_sample)  #[[2,0],[3,1]]
len(batch_sample)  #2

Sampler.py

import torch
from torch._six import int_classes as _int_classes


class Sampler(object):
    r"""Base class for all Samplers.

    Every Sampler subclass has to provide an :meth:`__iter__` method, providing a
    way to iterate over indices of dataset elements, and a :meth:`__len__` method
    that returns the length of the returned iterators.

    .. note:: The :meth:`__len__` method isn't strictly required by
              :class:`~torch.utils.data.DataLoader`, but is expected in any
              calculation involving the length of a :class:`~torch.utils.data.DataLoader`.
    """

    def __init__(self, data_source):
        pass

    def __iter__(self):
        raise NotImplementedError

    # NOTE [ Lack of Default `__len__` in Python Abstract Base Classes ]
    #
    # Many times we have an abstract class representing a collection/iterable of
    # data, e.g., `torch.utils.data.Sampler`, with its subclasses optionally
    # implementing a `__len__` method. In such cases, we must make sure to not
    # provide a default implementation, because both straightforward default
    # implementations have their issues:
    #
    #   + `return NotImplemented`:
    #     Calling `len(subclass_instance)` raises:
    #       TypeError: 'NotImplementedType' object cannot be interpreted as an integer
    #
    #   + `raise NotImplementedError()`:
    #     This prevents triggering some fallback behavior. E.g., the built-in
    #     `list(X)` tries to call `len(X)` first, and executes a different code
    #     path if the method is not found or `NotImplemented` is returned, while
    #     raising an `NotImplementedError` will propagate and and make the call
    #     fail where it could have use `__iter__` to complete the call.
    #
    # Thus, the only two sensible things to do are
    #
    #   + **not** provide a default `__len__`.
    #
    #   + raise a `TypeError` instead, which is what Python uses when users call
    #     a method that is not defined on an object.
    #     (@ssnl verifies that this works on at least Python 3.7.)


class SequentialSampler(Sampler):
    r"""Samples elements sequentially, always in the same order.

    Arguments:
        data_source (Dataset): dataset to sample from
    """

    def __init__(self, data_source):
        self.data_source = data_source

    def __iter__(self):
        return iter(range(len(self.data_source)))

    def __len__(self):
        return len(self.data_source)


class RandomSampler(Sampler):
    r"""Samples elements randomly. If without replacement, then sample from a shuffled dataset.
    If with replacement, then user can specify :attr:`num_samples` to draw.

    Arguments:
        data_source (Dataset): dataset to sample from
        replacement (bool): samples are drawn with replacement if ``True``, default=``False``
        num_samples (int): number of samples to draw, default=`len(dataset)`. This argument
            is supposed to be specified only when `replacement` is ``True``.
    """

    def __init__(self, data_source, replacement=False, num_samples=None):
        self.data_source = data_source
        self.replacement = replacement
        self._num_samples = num_samples

        if not isinstance(self.replacement, bool):
            raise ValueError("replacement should be a boolean value, but got "
                             "replacement={}".format(self.replacement))

        if self._num_samples is not None and not replacement:
            raise ValueError("With replacement=False, num_samples should not be specified, "
                             "since a random permute will be performed.")

        if not isinstance(self.num_samples, int) or self.num_samples <= 0:
            raise ValueError("num_samples should be a positive integer "
                             "value, but got num_samples={}".format(self.num_samples))

    @property
    def num_samples(self):
        # dataset size might change at runtime
        if self._num_samples is None:
            return len(self.data_source)
        return self._num_samples

    def __iter__(self):
        n = len(self.data_source)
        if self.replacement:
            return iter(torch.randint(high=n, size=(self.num_samples,), dtype=torch.int64).tolist())
        return iter(torch.randperm(n).tolist())

    def __len__(self):
        return self.num_samples


class SubsetRandomSampler(Sampler):
    r"""Samples elements randomly from a given list of indices, without replacement.

    Arguments:
        indices (sequence): a sequence of indices
    """

    def __init__(self, indices):
        self.indices = indices

    def __iter__(self):
        return (self.indices[i] for i in torch.randperm(len(self.indices)))

    def __len__(self):
        return len(self.indices)


class WeightedRandomSampler(Sampler):
    r"""Samples elements from ``[0,..,len(weights)-1]`` with given probabilities (weights).

    Args:
        weights (sequence)   : a sequence of weights, not necessary summing up to one
        num_samples (int): number of samples to draw
        replacement (bool): if ``True``, samples are drawn with replacement.
            If not, they are drawn without replacement, which means that when a
            sample index is drawn for a row, it cannot be drawn again for that row.

    Example:
        >>> list(WeightedRandomSampler([0.1, 0.9, 0.4, 0.7, 3.0, 0.6], 5, replacement=True))
        [0, 0, 0, 1, 0]
        >>> list(WeightedRandomSampler([0.9, 0.4, 0.05, 0.2, 0.3, 0.1], 5, replacement=False))
        [0, 1, 4, 3, 2]
    """

    def __init__(self, weights, num_samples, replacement=True):
        if not isinstance(num_samples, _int_classes) or isinstance(num_samples, bool) or \
                num_samples <= 0:
            raise ValueError("num_samples should be a positive integer "
                             "value, but got num_samples={}".format(num_samples))
        if not isinstance(replacement, bool):
            raise ValueError("replacement should be a boolean value, but got "
                             "replacement={}".format(replacement))
        self.weights = torch.as_tensor(weights, dtype=torch.double)
        self.num_samples = num_samples
        self.replacement = replacement

    def __iter__(self):
        return iter(torch.multinomial(self.weights, self.num_samples, self.replacement).tolist())

    def __len__(self):
        return self.num_samples


class BatchSampler(Sampler):
    r"""Wraps another sampler to yield a mini-batch of indices.

    Args:
        sampler (Sampler): Base sampler.
        batch_size (int): Size of mini-batch.
        drop_last (bool): If ``True``, the sampler will drop the last batch if
            its size would be less than ``batch_size``

    Example:
        >>> list(BatchSampler(SequentialSampler(range(10)), batch_size=3, drop_last=False))
        [[0, 1, 2], [3, 4, 5], [6, 7, 8], [9]]
        >>> list(BatchSampler(SequentialSampler(range(10)), batch_size=3, drop_last=True))
        [[0, 1, 2], [3, 4, 5], [6, 7, 8]]
    """

    def __init__(self, sampler, batch_size, drop_last):
        if not isinstance(sampler, Sampler):
            raise ValueError("sampler should be an instance of "
                             "torch.utils.data.Sampler, but got sampler={}"
                             .format(sampler))
        if not isinstance(batch_size, _int_classes) or isinstance(batch_size, bool) or \
                batch_size <= 0:
            raise ValueError("batch_size should be a positive integer value, "
                             "but got batch_size={}".format(batch_size))
        if not isinstance(drop_last, bool):
            raise ValueError("drop_last should be a boolean value, but got "
                             "drop_last={}".format(drop_last))
        self.sampler = sampler
        self.batch_size = batch_size
        self.drop_last = drop_last

    def __iter__(self):
        batch = []
        for idx in self.sampler:
            batch.append(idx)
            if len(batch) == self.batch_size:
                yield batch
                batch = []
        if len(batch) > 0 and not self.drop_last:
            yield batch

    def __len__(self):
        if self.drop_last:
            return len(self.sampler) // self.batch_size
        else:
            return (len(self.sampler) + self.batch_size - 1) // self.batch_size