What causes "Empty Instances does not support __len__!" in facebookresearch/detectron2?

Set at least one field before calling len(), e.g. instances.set('pred_boxes', Boxes(torch.zeros(0,4)))

facebookresearch/detectron2 · error · NotImplementedError

Empty Instances does not support len!

Error message

Empty Instances does not support __len__!

What it means

Instances.__len__ returns the length of the first field; with zero fields there is nothing to measure, so calling len() on an empty Instances raises NotImplementedError rather than guessing 0.

Solutions

Set at least one field before calling len(), e.g. instances.set('pred_boxes', Boxes(torch.zeros(0,4)))
Guard with len(instances.get_fields()) > 0 before calling len()
Use detectron2's empty_input context / handle the no-detection branch separately

Example fix

# before
n = len(instances)
# after
fields = instances.get_fields()
n = len(next(iter(fields.values()))) if fields else 0

Defensive patterns

Strategy: validation

Validate before calling

def instances_len(instances) -> int:
    fields = instances.get_fields()
    return len(next(iter(fields.values()))) if fields else 0

Prevention

Never call len() on possibly-empty Instances
Initialize fields (e.g. empty Boxes) before length checks

When it happens

Trigger: len(Instances(image_size)) right after construction, or on an Instances whose fields were never set (e.g. empty detections built manually).

Common situations: Postprocessing code doing len(predictions['instances']) before any head populated fields; iterating results of a model with all heads disabled.

Related errors

AI-assisted analysis of facebookresearch/detectron2@a2f4a8771a (2026-08-27). Data as JSON: /api/errors/098defb28426a5c8. Report an issue: GitHub.

Appendix: source

Thrown at detectron2/structures/instances.py:148

            If `item` is a string, return the data in the corresponding field.
            Otherwise, returns an `Instances` where all fields are indexed by `item`.
        """
        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]

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