{"record":{"id":"4aa5108adf6a0cfe","repo":"apache/beam","slug":"unable-to-find-batchconverter-for-element-type-element-type","errorCode":null,"errorMessage":"Unable to find BatchConverter for element_type={element_type!r} and batch_type={batch_type!r}. Error summaries:\n\n{error_summaries}","messagePattern":"Unable to find BatchConverter for element_type=(.+?) and batch_type=(.+?)\\. Error summaries:\n\n(.+?)","errorType":"validation","errorClass":"TypeError","httpStatus":null,"severity":"error","filePath":"sdks/python/apache_beam/typehints/batch.py","lineNumber":101,"sourceCode":"      BATCH_CONVERTER_REGISTRY[name] = batch_converter_constructor\n      return batch_converter_constructor\n\n    return do_registration\n\n  @staticmethod\n  def from_typehints(*, element_type, batch_type) -> 'BatchConverter':\n    element_type = typehints.normalize(element_type)\n    batch_type = typehints.normalize(batch_type)\n    errors = {}\n    for name, constructor in BATCH_CONVERTER_REGISTRY.items():\n      try:\n        return constructor(element_type, batch_type)\n      except TypeError as e:\n        errors[name] = e.args[0]\n\n    error_summaries = '\\n\\n'.join(\n        f\"{name}:\\n\\t{msg}\" for name, msg in errors.items())\n    raise TypeError(\n        f\"Unable to find BatchConverter for element_type={element_type!r} and \"\n        f\"batch_type={batch_type!r}. Error summaries:\\n\\n{error_summaries}\")\n\n  @property\n  def batch_type(self):\n    return self._batch_type\n\n  @property\n  def element_type(self):\n    return self._element_type\n\n  def __key(self):\n    return (self._element_type, self._batch_type)\n\n  def __eq__(self, other: 'BatchConverter') -> bool:\n    if isinstance(other, BatchConverter):\n      return self.__key() == other.__key()\n","sourceCodeStart":83,"sourceCodeEnd":119,"githubUrl":"https://github.com/apache/beam/blob/12126d8942aaf848030c478b4c6a28c6af861c66/sdks/python/apache_beam/typehints/batch.py#L83-L119","documentation":"BatchConverter.from_typehints tries every registered BatchConverter constructor for the given element_type/batch_type pair; if all raise TypeError, it aggregates their messages and raises this TypeError. It means no registered converter can convert between the two type hints.","triggerScenarios":"Calling BatchConverter.from_typehints(element_type, batch_type) where no registered converter (list, numpy, pandas, arrow, etc.) accepts the pair, e.g. from_typehints(int, Dict[str, int]) or an unregistered custom class as batch type.","commonSituations":"Typo'd or mismatched type hints in BatchElements/CollapseBatches pipelines; using a batch type no converter is registered for; forgetting to import apache_beam.dataframe or pandas convertors so no constructor matches.","solutions":["Check registered converters and pass a matching pair, e.g. from_typehints(int, List[int]) for the list converter","Verify the batch_type parameterization matches element_type exactly (List[T] with same T)","Import the module that registers the converter you need (e.g. pandas/arrow converters) before calling","Register a custom BatchConverter for your element/batch type pair"],"exampleFix":"// before\nBatchConverter.from_typehints(int, Dict[str, int])\n// after\nBatchConverter.from_typehints(int, List[int])","handlingStrategy":"try-catch","validationCode":"from apache_beam.typehints import batch, typehints\nif not isinstance(batch_type, typehints.ListConstraint):\n    raise ValueError('use List[T] as batch type')","typeGuard":"from apache_beam.typehints import typehints\ndef is_valid_batch_pair(elem_t, batch_t):\n    return isinstance(batch_t, typehints.ListConstraint) and batch_t.inner_type == elem_t","tryCatchPattern":"try:\n    conv = BatchConverter.from_typehints(elem_t, batch_t)\nexcept TypeError as e:\n    print(e)  # includes per-converter error summaries; pick a supported pair","preventionTips":["Pair each element type with its documented batch type (List[T], np.ndarray, pd.DataFrame)","Import converter-registering modules before calling from_typehints","Read the aggregated error_summaries in the message to see why each converter rejected the pair"],"tags":["python","apache-beam","typehints","batching"],"backgroundTag":"type-mismatch","analyzedSha":"12126d8942aaf848030c478b4c6a28c6af861c66","analyzedAt":"2026-09-13T01:50:10.254Z","contentChangedAt":"2026-09-13T01:50:10.254Z","schemaVersion":2},"datasetVersion":"2026-09-14T21:17:11.552Z"}