apache/beam · error · IllegalArgumentException

Unrecognized input type

Error message

Unrecognized input type: ${input}

What it means

YamlTransform.expand only knows how to feed an external transform from a PCollection or a PCollectionRowTuple. If the expand() input is any other PValue/PCollection type, the library cannot map it onto the external transform's input and throws IllegalArgumentException naming the object's type.

Solutions

  1. Convert the input to a PCollection or PCollectionRowTuple before applying the YamlTransform.
  2. If the input is a PCollectionList, flatten or index it into a single PCollection first.
  3. Use a standard (non-YAML) transform if you genuinely need exotic input types.
  4. Check the message's printed object for the concrete class and adjust the pipeline shape accordingly.

Example fix

// before
PCollectionList pcs = ...; pipeline.apply(yamlTransform, pcs);
// after
pcs.get(0).apply(yamlTransform);
Defensive patterns

Strategy: type-guard

Validate before calling

if (!(input instanceof PCollection) && !(input instanceof PCollectionRowTuple)) {
  throw new IllegalArgumentException("YamlTransform requires PCollection or PCollectionRowTuple, got " + input.getClass());
}

Type guard

boolean isYamlTransformInput(PValue v) {
  return v instanceof PCollection || v instanceof PCollectionRowTuple;
}

Try / catch

try {
  return yamlTransform.expand(input);
} catch (IllegalArgumentException e) {
  if (e.getMessage().startsWith("Unrecognized input type")) { /* convert input */ }
  throw e;
}

Prevention

When it happens

Trigger: Passing a PCollectionList, PCollectionTuple, or other PValue subtype as the input element of a YamlTransform in expand().

Common situations: Building multi-input pipelines where intermediate values are PCollectionList (e.g. from Flatten/Partition outputs) and mistakenly passing them to a YAML transform; refactorings that change input types.

Understand the failure class

Background: "is not a compatible type" / "cannot merge" errors: when a value's type doesn't match what the library requires — this error's family across 65 libraries.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/485f27ff3ac083d2. Report an issue: GitHub.

Appendix: source

Thrown at sdks/java/extensions/yaml/src/main/java/org/apache/beam/sdk/extensions/yaml/YamlTransform.java:170

    }

    // There is no generic apply...
    POutput output;
    @SuppressWarnings("rawtypes")
    PTransform externalTransform =
        PythonExternalTransform.from("apache_beam.yaml.yaml_transform.YamlTransform")
            .withArgs(yamlDefinition)
            .withExtraPackages(ImmutableList.of("jinja2", "pyyaml", "virtualenv-clone"));
    if (input instanceof PBegin) {
      output = ((PBegin) input).apply(externalTransform);
    } else if (input instanceof PCollection) {
      output = ((PCollection<?>) input).apply(externalTransform);
    } else if (input instanceof PCollection) {
      output = ((PCollection<?>) input).apply(externalTransform);
    } else if (input instanceof PCollectionRowTuple) {
      output = ((PCollectionRowTuple) input).apply(externalTransform);
    } else {
      throw new IllegalArgumentException("Unrecognized input type: " + input);
    }

    if (outputTags == null) {
      if (!(output instanceof PCollection)) {
        throw new IllegalArgumentException(
            "Expected a single PCollection output, but got "
                + output
                + ". Perhaps withMultipleOutputs() needs to be specified?");
      }
      return (OutputT) output;
    } else {
      if (output instanceof PCollection) {
        // ExternalPythonTransform always returns single outputs as PCollections.
        if (outputTags.size() != 1) {
          throw new IllegalArgumentException(
              "Expected " + outputTags.size() + " outputs, but got exactly one.");
        }
        return (OutputT)

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