apache/beam · error · ValueError

Expansion service with target

Error message

Expansion service with target '{target}' does not specify any default destinations.

What it means

gen_xlang_wrappers.py reads the expansion-services YAML config and, for each service entry, requires a 'destinations' mapping of SDK to output directory. A service without that key cannot have wrappers generated, so generate_transforms_config raises ValueError naming the service's gradle target.

Solutions

Add a 'destinations' mapping (SDK -> directory) to the offending service entry in the expansion services YAML.
Match the destinations format of existing entries and confirm each destination directory exists under sdks/python.
Re-run gen_xlang_wrappers.py.

Example fix

// before (services yaml)
- gradle_target: :sdks:java:io:expansion-service:shadowJar
// after
- gradle_target: :sdks:java:io:expansion-service:shadowJar
  destinations:
    python: sdks/python/apache_beam/transforms/xlang_io

Defensive patterns

Strategy: validation

Validate before calling

for svc in yaml.safe_load(open(services_yaml)):
    assert 'destinations' in svc, f"service {svc.get('gradle_target')} needs a 'destinations' map"

Type guard

def has_destinations(svc):
    return isinstance(svc, dict) and isinstance(svc.get('destinations'), dict) and bool(svc['destinations'])

Try / catch

try:
    generate_transforms_config(services_yaml, ...)
except ValueError as e:
    if 'does not specify any default destinations' in str(e):
        print('Add destinations to the offending expansion service entry')

Prevention

Validate the services YAML schema in CI before running generation.
Copy the structure of existing service entries when registering new ones.
Keep a JSON schema for the expansion services config.

When it happens

Trigger: An expansion service entry in the input services YAML lacks the required 'destinations' field while running the xlang wrapper generation script.

Common situations: Contributors registering a new expansion service forget to add default destinations; YAML restructuring accidentally drops the key.

Understand the failure class

Background: "is required", "must be set", "missing required field": configuration validation errors across open-source libraries — this error's family across 36 libraries.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/108535e5f8cb9fa4. Report an issue: GitHub.

Appendix: source

Thrown at sdks/python/gen_xlang_wrappers.py:126

    - gradle_target: 'sdks:java:io:expansion-service:shadowJar'
      destinations:
        python: 'apache_beam/io'
      skip_transforms:
        - 'beam:schematransform:org.apache.beam:some_transform:v1'
  """
  from apache_beam.transforms.external import BeamJarExpansionService
  from apache_beam.transforms.external_transform_provider import ExternalTransform
  from apache_beam.transforms.external_transform_provider import ExternalTransformProvider

  transform_list: list[dict[str, Any]] = []

  with open(input_services) as f:
    services = yaml.safe_load(f)
  for service in services:
    target = service['gradle_target']

    if "destinations" not in service:
      raise ValueError(
          f"Expansion service with target '{target}' does not "
          "specify any default destinations.")
    service_destinations: dict[str, str] = service['destinations']
    for sdk, dest in service_destinations.items():
      validate_sdks_destinations(sdk, dest, target)

    transforms_to_skip = service.get('skip_transforms', [])

    # use dynamic provider to discover and populate wrapper details
    provider = ExternalTransformProvider(BeamJarExpansionService(target))
    discovered: dict[str, ExternalTransform] = provider.get_all()
    for identifier in sorted(discovered.keys()):
      wrapper = discovered[identifier]
      if identifier in transforms_to_skip:
        continue

      transform_destinations = service_destinations.copy()

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