{"record":{"id":"25534dbd07ac4cb3","repo":"apache/beam","slug":"encountered-an-empty-schema","errorCode":null,"errorMessage":"Encountered an empty schema","messagePattern":"Encountered an empty schema","errorType":"validation","errorClass":"ValueError","httpStatus":null,"severity":"error","filePath":"sdks/python/apache_beam/io/gcp/bigquery_schema_tools.py","lineNumber":78,"sourceCode":"    type_overrides=None) -> type:\n  \"\"\"Convert a schema of type TableSchema into a pcollection element.\n\n  Args:\n    the_table_schema: A BQ schema of type TableSchema\n    selected_fields: if not None, the subset of fields to consider\n    type_overrides: Optional mapping of BigQuery type names (uppercase)\n      to Python types. These override the default mappings in\n      BIG_QUERY_TO_PYTHON_TYPES. For example:\n      ``{'DATE': datetime.date, 'JSON': dict}``\n\n  Returns:\n    type: type that can be used to work with pCollections.\n  \"\"\"\n  effective_types = {**BIG_QUERY_TO_PYTHON_TYPES, **(type_overrides or {})}\n  the_schema = beam.io.gcp.bigquery_tools.get_dict_table_schema(\n      the_table_schema)\n  if the_schema == {}:\n    raise ValueError(\"Encountered an empty schema\")\n  field_names_and_types = []\n  for field in the_schema['fields']:\n    if selected_fields is not None and field['name'] not in selected_fields:\n      continue\n    if field['type'] in effective_types:\n      typ = bq_field_to_type(field['type'], field['mode'], type_overrides)\n    else:\n      raise ValueError(\n          f\"Encountered \"\n          f\"an unsupported type: {field['type']!r}\")\n    field_names_and_types.append((field['name'], typ))\n  sample_schema = beam.typehints.schemas.named_fields_to_schema(\n      field_names_and_types)\n  usertype = beam.typehints.schemas.named_tuple_from_schema(sample_schema)\n  return usertype\n\n\ndef bq_field_to_type(field, mode, type_overrides=None):","sourceCodeStart":60,"sourceCodeEnd":96,"githubUrl":"https://github.com/apache/beam/blob/12126d8942aaf848030c478b4c6a28c6af861c66/sdks/python/apache_beam/io/gcp/bigquery_schema_tools.py#L60-L96","documentation":"generate_user_type_from_bq_schema converts a BigQuery table schema into a Python user type for typed reads. It first normalizes the schema via get_dict_table_schema; if the result is an empty dict (no fields), there is nothing to build a row type from, so it raises this ValueError.","triggerScenarios":"Calling generate_user_type_from_bq_schema (or convert_to_usertype) with a table schema whose fields list is empty, or a TableSchema that serializes to {}.","commonSituations":"Reading from a query returning no columns (e.g. SELECT from an empty-defined view), passing an unpopulated TableSchema() constructed programmatically, or a metadata fetch returning no fields due to permission/table-not-found issues.","solutions":["Verify the source table/query schema is non-empty before calling; run the query or table.get and check schema.fields.","If constructing TableSchema manually, populate at least one TableFieldSchema.","Pass an explicit the_table_schema from a known-good table instead of a dynamically fetched empty one."],"exampleFix":"// before\nif not the_table_schema:\n  the_table_schema = TableSchema()\n// after\nif not the_table_schema or not the_table_schema.fields:\n  raise ValueError('table schema must define at least one field')","handlingStrategy":"validation","validationCode":"d = beam.io.gcp.bigquery_tools.get_dict_table_schema(the_table_schema)\nif not d or not d.get('fields'):\n    raise ValueError('table schema has no fields; cannot build user type')","typeGuard":"def has_fields(table_schema) -> bool:\n    return bool(table_schema and getattr(table_schema, 'fields', None))","tryCatchPattern":"try:\n    user_type = bigquery_schema_tools.generate_user_type_from_bq_schema(schema)\nexcept ValueError as e:\n    if 'empty schema' in str(e):\n        fetch_schema_from_table()","preventionTips":["Fetch the table schema from BigQuery before typed reads","Check schema.fields is non-empty for programmatic TableSchema objects","Handle empty query results / missing table permissions upstream"],"tags":["bigquery","schema","beam","validation"],"backgroundTag":"schema-validation-failed","analyzedSha":"12126d8942aaf848030c478b4c6a28c6af861c66","analyzedAt":"2026-09-13T01:50:10.254Z","contentChangedAt":"2026-09-13T01:50:10.254Z","schemaVersion":2},"datasetVersion":"2026-09-14T16:17:12.679Z"}