apache/beam · error · ValueError

Unknown language for mapping transform

Error message

Unknown language for mapping transform: {language}. Supported languages are "javascript" and "python."

What it means

`_as_callable` supports only language values 'javascript', 'python', 'generic', or unset (None). Any other `language` value in the transform config reaches this ValueError listing the supported languages.

Solutions

1. Set language to exactly 'javascript' or 'python' (or omit it entirely for the default Python behavior).
2. Fix casing and spelling: 'js' and 'JavaScript' are not accepted.
3. If you need another language, implement the UDF in Python or JavaScript instead, or extend the transform.

Example fix

# before
config:
  language: js
# after
config:
  language: javascript

assert language in (None, 'python', 'generic', 'javascript'), f'unsupported language {language!r}'

def is_supported_language(lang) -> bool:
    return lang in (None, 'python', 'generic', 'javascript')

try:
    run_pipeline(yaml_spec)
except ValueError as e:
    if 'Unknown language for mapping transform' in str(e):
        yaml_spec['config']['language'] = normalize_language(yaml_spec['config']['language'])
        run_pipeline(yaml_spec)
    else:
        raise

When it happens

Trigger: Setting `language: js`, `language: java`, `language: Python` (case-sensitive — 'Python' with a capital P also fails? no: it compares exact strings, so 'Python' fails), or any typo'd language name in MapToFields/other mapping-transform config.

Common situations: Typos ('java' vs 'javascript', 'js' abbreviation), wrong casing ('Python'), or assuming other UDF languages (Go, SQL, Java) are supported in YAML mapping transforms.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

• Ambiguous expression type (perhaps missing quoting?)
• CombineFn spec missing type
• cross_product must be specified true or false when…
• Missing combine parameter in Combine config.
• Unknown CombineFn

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/eb0eeed6a944b83a. Report an issue: GitHub.

Appendix: source

Thrown at sdks/python/apache_beam/yaml/yaml_mapping.py:413

  # Extract original type from upstream pcoll when doing simple mappings
  original_type = input_schema.get(expr.get('expression'), None)
  if expr in original_fields:
    language = "python"

  # TODO(yaml): support an imports parameter
  # TODO(yaml): support a requirements parameter (possibly at a higher level)
  if not isinstance(expr, dict):
    raise ValueError(
        f"Ambiguous expression type (perhaps missing quoting?): {expr}")
  explicit_type = expr.pop('output_type', None)
  _check_mapping_arguments(transform_name, **expr)

  if language == "javascript":
    func = _expand_javascript_mapping_func(original_fields, **expr)
  elif language in ("python", "generic", None):
    func = _expand_python_mapping_func(original_fields, **expr)
  else:
    raise ValueError(
        f'Unknown language for mapping transform: {language}. '
        'Supported languages are "javascript" and "python."')

  if explicit_type:
    if isinstance(explicit_type, str):
      explicit_type = {'type': explicit_type}
    beam_type = json_utils.json_type_to_beam_type(explicit_type)
    validator = _validator(beam_type)

    @beam.typehints.with_output_types(schemas.typing_from_runner_api(beam_type))
    def checking_func(row):
      result = func(row)
      if not validator(result):
        raise TypeError(f'{result} violates schema {explicit_type}')
      return result

    return checking_func

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