apache/beam · error · TypeError

f'{attr_type} of test specification {identifier} must be a l

Error message

f'{attr_type} of test specification {identifier} must be a list, got {type(attr_type)}'

What it means

A test specification attribute that must be a list of named element groups (mock_inputs, mock_outputs, expected_outputs, or expected_inputs) was supplied as some other type. validate_test_spec iterates these four attributes and this guard fires on the first one that is not a Python list, naming the offending attr_type and test identifier. Note the message interpolates type(attr_type) (the attribute name's type) rather than the actual value's type, so it always reports <class 'str'>.

Source

Thrown at sdks/python/apache_beam/yaml/yaml_testing.py:179

          'name',
          'mock_inputs',
          'mock_outputs',
          'expected_outputs',
          'expected_inputs',
          'allowed_sources',
      ])
  if unknown_attrs:
    raise ValueError(
        f'test specification {identifier} '
        f'has unknown attributes {list(unknown_attrs)}')

  for attr_type in ('mock_inputs',
                    'mock_outputs',
                    'expected_outputs',
                    'expected_inputs'):
    attr = test_spec.get(attr_type, [])
    if not isinstance(attr, list):
      raise TypeError(
          f'{attr_type} of test specification {identifier} '
          f'must be a list, got {type(attr_type)}')
    for ix, attr_item in enumerate(attr):
      if not isinstance(attr_item, dict):
        raise TypeError(
            f'{attr_type} {ix} of test specification {identifier} '
            f'must be an object, got {type(attr_item)}')
      if 'name' not in attr_item:
        raise TypeError(
            f'{attr_type} {ix} of test specification {identifier} '
            f'missing a name')
      if 'elements' not in attr_item:
        raise TypeError(
            f'{attr_type} {ix} of test specification {identifier} '
            f'missing a elements')
      if not isinstance(attr_item['elements'], list):
        raise TypeError(
            f'{attr_type} {ix} of test specification {identifier} '

View on GitHub (pinned to 12126d8942)

Solutions

1. Set the offending attribute (e.g. mock_inputs) to a YAML/Python list of mappings, each with 'name' and 'elements' keys
2. If only one entry is needed, still wrap it in a list: mock_inputs: [{name: ..., elements: [...]}]
3. Check for YAML indentation errors that made the attribute parse as a scalar or map instead of a sequence

When it happens

Trigger: Thrown at sdks/python/apache_beam/yaml/yaml_testing.py:179 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/5ef63b8028925162. Report an issue: GitHub.