**sampler.py:**

```
class ImbalancedDatasetSampler(Sampler):
"""Samples elements randomly from a given list of indices for imbalanced dataset
Arguments:
indices (list, optional): a list of indices
num_samples (int, optional): number of samples to draw
"""
def __init__(self, dataset, indices=None, num_samples=None):
# if indices is not provided,
# all elements in the dataset will be considered
self.indices = list(range(len(dataset))) \
if indices is None else indices
# if num_samples is not provided,
# draw `len(indices)` samples in each iteration
self.num_samples = len(self.indices) \
if num_samples is None else num_samples
# distribution of classes in the dataset
label_to_count = {}
for idx in self.indices:
label = self._get_label(dataset, idx)
if label in label_to_count:
label_to_count[label] += 1
else:
label_to_count[label] = 1
# weight for each sample
weights = [1.0 / label_to_count[self._get_label(dataset, idx)]
for idx in self.indices]
self.weights = torch.DoubleTensor(weights)
def _get_label(self, dataset, idx):
dataset_type = type(dataset)
if dataset_type is torchvision.datasets.MNIST:
return dataset.train_labels[idx].item()
elif dataset_type is torchvision.datasets.ImageFolder:
return dataset.imgs[idx][1]
else:
raise NotImplementedError
def __iter__(self):
return (self.indices[i] for i in torch.multinomial(
self.weights, self.num_samples, replacement=True))
def __len__(self):
return self.num_samples
```

**dataloader.py (in DataLoader class):**

`from . import SequentialSampler, RandomSampler, BatchSampler, ImbalancedDatasetSampler`

&

```
# sampler = RandomSampler(dataset)
sampler = ImbalancedDatasetSampler(dataset)
```

**init.py**

`from .sampler import Sampler, SequentialSampler, RandomSampler, SubsetRandomSampler, WeightedRandomSampler, BatchSampler, ImbalancedDatasetSampler`