facebookresearch/detectron2 · error · NotImplementedError

`Instances` object is not iterable!

Error message

`Instances` object is not iterable!

What it means

Instances deliberately does not implement __iter__ because per-instance iteration is ambiguous over which field drives the length; iterating with a for loop raises NotImplementedError.

Source

Thrown at detectron2/structures/instances.py:151

        if type(item) is int:
            if item >= len(self) or item < -len(self):
                raise IndexError("Instances index out of range!")
            else:
                item = slice(item, None, len(self))

        ret = Instances(self._image_size)
        for k, v in self._fields.items():
            ret.set(k, v[item])
        return ret

    def __len__(self) -> int:
        for v in self._fields.values():
            # use __len__ because len() has to be int and is not friendly to tracing
            return v.__len__()
        raise NotImplementedError("Empty Instances does not support __len__!")

    def __iter__(self):
        raise NotImplementedError("`Instances` object is not iterable!")

    @staticmethod
    def cat(instance_lists: List["Instances"]) -> "Instances":
        """
        Args:
            instance_lists (list[Instances])

        Returns:
            Instances
        """
        assert all(isinstance(i, Instances) for i in instance_lists)
        assert len(instance_lists) > 0
        if len(instance_lists) == 1:
            return instance_lists[0]

        image_size = instance_lists[0].image_size
        if not isinstance(image_size, torch.Tensor):  # could be a tensor in tracing
            for i in instance_lists[1:]:

View on GitHub (pinned to a2f4a8771a)

Solutions

1. Index explicitly: for i in range(len(instances)): use instances.pred_classes[i], instances.scores[i]
2. Convert to rows: instances_to_coco_json(instances, img_id) or zip(*fields.values())
3. Wrap in a helper that materializes per-instance dicts once

Example fix

# before
for inst in instances:
    print(inst.scores)
# after
for i in range(len(instances)):
    print(instances.scores[i])

When it happens

Trigger: `for inst in instances:` or iter(instances) on any Instances object, populated or not.

Common situations: Treating Instances like a list of per-instance dicts (as some older Detectron1 or torchvision APIs allow); list comprehension over detections.

Related errors

• Cannot find field '{}' in the given Instances!
• Instances index out of range!
• Empty Instances does not support __len__!
• Unsupported type {} for concatenation
• Cannot match one checkpoint key to multiple keys in the mode

AI-assisted analysis of facebookresearch/detectron2@a2f4a8771a (2026-08-27). Data as JSON: /api/errors/5b6cb79e08f957bf. Report an issue: GitHub.