HumanSignal/label-studio · error · ValidationError

list should not be empty

Error message

list should not be empty

What it means

DRF ValidationError from the validate_string_list field validator on ML ModelInterface, raised when the value is falsy (empty list, None, or empty string). The field must be a non-empty list of strings.

Source

Thrown at label_studio/ml_models/models.py:25

from django.db.models import Q
from django.utils.translation import gettext_lazy as _
from ml_model_providers.models import ModelProviderConnection, ModelProviders
from projects.models import Project
from rest_framework.exceptions import ValidationError
from tasks.models import Annotation, FailedPrediction, Prediction, PredictionMeta

logger = logging.getLogger(__name__)


# skills are partitions of projects (label config + input columns + output columns) into categories of labeling tasks
class SkillNames(models.TextChoices):
    TEXT_CLASSIFICATION = 'TextClassification', _('TextClassification')
    NAMED_ENTITY_RECOGNITION = 'NamedEntityRecognition', _('NamedEntityRecognition')


def validate_string_list(value):
    if not value:
        raise ValidationError('list should not be empty')
    if not isinstance(value, list):
        raise ValidationError('Value must be a list')
    if not all(isinstance(item, str) for item in value):
        raise ValidationError('All items in the list must be strings')


class ModelInterface(models.Model):
    title = models.CharField(_('title'), max_length=500, null=False, blank=False, help_text='Model name')

    description = models.TextField(_('description'), null=True, blank=True, help_text='Model description')

    created_by = models.ForeignKey(
        settings.AUTH_USER_MODEL, related_name='created_models', on_delete=models.SET_NULL, null=True
    )

    created_at = models.DateTimeField(_('created at'), auto_now_add=True)

    updated_at = models.DateTimeField(_('updated at'), auto_now=True)

View on GitHub (pinned to 0b49e9b539)

Solutions

1. Provide a non-empty list value for the field
2. Fix the ML backend to always return its full interface spec
3. If empty should be legal, remove/replace the validator or make the field optional in the serializer

Example fix

// before
{"labels": []}
// after
{"labels": ["class_a", "class_b"]}

if not value:
    raise ValueError('field must be a non-empty list before submitting the model interface')

def is_nonempty_str_list(v) -> bool:
    return isinstance(v, list) and len(v) > 0

try:
    interface.save()
except ValidationError as e:
    if 'list should not be empty' in str(e):
        interface.labels = fetch_default_labels_from_backend()
        interface.save()

When it happens

Trigger: Saving or validating a ModelInterface whose list field (e.g. model controls/config lists) is [], None, or ''.

Common situations: ML backend returned an empty spec; client POSTed an omitted/empty field; deserialization defaulted to empty list instead of a required value.

Related errors

• Value must be a list
• All items in the list must be strings
• required
• FSMStateField is read-only. Use transitions to change state.
• {connection validation error}

AI-assisted analysis of HumanSignal/label-studio@0b49e9b539 (2026-08-29). Data as JSON: /api/errors/71f4feb86a0a68a4. Report an issue: GitHub.