HumanSignal/label-studio · error · ValidationError

replace(old_value, new_value) requires two arguments.

Error message

replace(old_value, new_value) requires two arguments.

What it means

The DataManager 'replace' expression substitutes occurrences of old_value with new_value in a column. add_expression enforces exactly two JSON-encoded arguments and raises this ValidationError when len(args) != 2.

Source

Thrown at label_studio/data_manager/actions/data_columns.py:310

    elif command == 'sample':
        if args:
            raise ValidationError({'value': 'sample() does not accept arguments.'})
        sample_values = iter(random.sample(range(0, size), size))

    elif command == 'random':
        if len(args) != 2:
            raise ValidationError({'value': 'random(min, max) requires two arguments.'})
        minimum, maximum = int(args[0]), int(args[1])

    elif command == 'choices':
        if not 0 < len(args) < 3:
            raise ValidationError({'value': 'choices(values:list, weights:list) requires one or two arguments.'})
        weights = json.loads(args[1]) if len(args) == 2 else None
        choices = json.loads(args[0])

    elif command == 'replace':
        if len(args) != 2:
            raise ValidationError({'value': 'replace(old_value, new_value) requires two arguments.'})
        old_value, new_value = json.loads(args[0]), json.loads(args[1])

    else:
        raise ValidationError('Undefined expression, you can use: ' + add_data_field_examples)

    offset = 0
    task_iterator = iterate_queryset(queryset.only('id', 'data'), chunk_size=settings.UPDATE_COLUMN_BATCH_SIZE)
    for task_batch in batched_iterator(task_iterator, settings.UPDATE_COLUMN_BATCH_SIZE):
        if command == 'range':
            for index, task in enumerate(task_batch):
                task.data[value_name] = start + offset + index
        elif command == 'sample':
            for task in task_batch:
                task.data[value_name] = next(sample_values)
        elif command == 'random':
            for task in task_batch:
                task.data[value_name] = random.randint(minimum, maximum)
        elif command == 'choices':

View on GitHub (pinned to 0b49e9b539)

Solutions

1. Provide exactly two JSON-encoded arguments: {"args": ["\"foo\"", "\"bar\""]}
2. To replace with empty string, pass the JSON string "" as new_value rather than omitting the argument
3. Ensure old_value and new_value round-trip through json.loads (quoted strings, numbers, etc.)

Example fix

// before
{"command": "replace", "args": ["\"foo\""]}
// after
{"command": "replace", "args": ["\"foo\"", "\"bar\""]}

if (expr.command === 'replace' && (!Array.isArray(expr.args) || expr.args.length !== 2)) {
  throw new Error('replace requires exactly [old_value, new_value]');
}

function isValidReplace(e) { return e.command === 'replace' && Array.isArray(e.args) && e.args.length === 2; }

try {
  await dm.addAction({id: 'add_data_field', expression: expr});
} catch (e) {
  if (String(e).includes('replace(old_value, new_value) requires two')) {
    expr.args = [JSON.stringify(oldV), JSON.stringify(newV)];
    return retry();
  }
  throw e;
}

When it happens

Trigger: Calling the add-data-field action with {"command": "replace", "args": ["foo"]} (missing new value), args: [], or three+ args via the column mutation action.

Common situations: Developers pass only the old value assuming a delete-replace; arguments not JSON-encoded (plain strings like foo instead of "foo") so they are dropped or malformed; UI sending empty optional fields.

Related errors

• sample() does not accept arguments.
• random(min, max) requires two arguments.
• choices(values:list, weights:list) requires one or two argum
• Undefined expression, you can use: {add_data_field_examples}
• Can't find '{action_id}' in registered actions

AI-assisted analysis of HumanSignal/label-studio@0b49e9b539 (2026-08-29). Data as JSON: /api/errors/f07a8fe9ae9290ff. Report an issue: GitHub.