WZMIAOMIAO/deep-learning-for-image-processing · error · ValueError

sampler should be an instance of torch.utils.data.Sampler, b

Error message

sampler should be an instance of torch.utils.data.Sampler, but got sampler={}

What it means

GroupedBatchSampler.__init__ requires `sampler` to be an instance of torch.utils.data.Sampler; anything else raises ValueError. group_ids must also be a continuous integer range [0, num_groups).

Source

Thrown at pytorch_object_detection/faster_rcnn/train_utils/group_by_aspect_ratio.py:39

class GroupedBatchSampler(BatchSampler):
    """
    Wraps another sampler to yield a mini-batch of indices.
    It enforces that the batch only contain elements from the same group.
    It also tries to provide mini-batches which follows an ordering which is
    as close as possible to the ordering from the original sampler.
    Arguments:
        sampler (Sampler): Base sampler.
        group_ids (list[int]): If the sampler produces indices in range [0, N),
            `group_ids` must be a list of `N` ints which contains the group id of each sample.
            The group ids must be a continuous set of integers starting from
            0, i.e. they must be in the range [0, num_groups).
        batch_size (int): Size of mini-batch.
    """
    def __init__(self, sampler, group_ids, batch_size):
        if not isinstance(sampler, Sampler):
            raise ValueError(
                "sampler should be an instance of "
                "torch.utils.data.Sampler, but got sampler={}".format(sampler)
            )
        self.sampler = sampler
        self.group_ids = group_ids
        self.batch_size = batch_size

    def __iter__(self):
        buffer_per_group = defaultdict(list)
        samples_per_group = defaultdict(list)

        num_batches = 0
        for idx in self.sampler:
            group_id = self.group_ids[idx]
            buffer_per_group[group_id].append(idx)
            samples_per_group[group_id].append(idx)
            if len(buffer_per_group[group_id]) == self.batch_size:
                yield buffer_per_group[group_id]

View on GitHub (pinned to 1ec3fe6f37)

Solutions

1. Wrap the indices in torch.utils.data.sampler.SequentialSampler or RandomSampler before passing
2. If using a custom sampler, make it inherit from torch.utils.data.Sampler and implement __iter__/__len__
3. Check you did not accidentally pass the dataset or batch_size object as the first argument

Example fix

// before
sampler = list(range(len(dataset)))
gbs = GroupedBatchSampler(sampler, group_ids, batch_size=8)
// after
from torch.utils.data import SequentialSampler
sampler = SequentialSampler(dataset)
gbs = GroupedBatchSampler(sampler, group_ids, batch_size=8)

from torch.utils.data import Sampler
assert isinstance(sampler, Sampler), "sampler must be a torch.utils.data.Sampler"
gbs = GroupedBatchSampler(sampler, group_ids, batch_size=8)

from torch.utils.data import Sampler
def is_sampler(s) -> bool:
    return isinstance(s, Sampler)

try:
    gbs = GroupedBatchSampler(sampler, group_ids, batch_size)
except ValueError as e:
    if "instance of" in str(e):
        from torch.utils.data import SequentialSampler
        gbs = GroupedBatchSampler(SequentialSampler(dataset), group_ids, batch_size)

When it happens

Trigger: Passing a plain list, range, generator, or a function as `sampler` instead of a Sampler instance, e.g. GroupedBatchSampler(list(range(100)), group_ids, batch_size).

Common situations: Confusing sampler with the underlying indices; passing a dataset or DataLoader instead of a sampler; custom samplers not inheriting from torch.utils.data.Sampler after a refactor.

Understand the failure class

Background: Invalid argument type errors: "must be of type string", "expected X, got Y", and ERR_INVALID_ARG_TYPE explained — this error's family across 15 libraries.

Related errors

• sampler should be an instance of torch.utils.data.Sampler, b
• expected stages_repeats as list of 3 positive ints
• expected stages_out_channels as list of 5 positive ints
• image: {} isn't RGB mode.
• dataset have {} classes, but input {}

AI-assisted analysis of WZMIAOMIAO/deep-learning-for-image-processing@1ec3fe6f37 (2026-08-30). Data as JSON: /api/errors/acf2ac8b5bd5ee81. Report an issue: GitHub.