What causes "panic(formatParDoError(dofn, len(ret), 5))" in apache/beam?

Update the DoFn's ProcessElement to emit exactly 5 outputs.

panic(formatParDoError(dofn, len(ret), 5))

Error message

panic(formatParDoError(dofn, len(ret), 5))

What it means

Panic raised by the typed ParDo5 convenience wrapper when the underlying TryParDo returned a number of output PCollections other than 5 (as reported by formatParDoError). It means the DoFn's emitted outputs (or the options given) did not match the fixed 5-output arity this helper promises; the panic occurs at pipeline construction, not at runtime.

Source

Thrown at sdks/go/pkg/beam/pardo.go:469

		panic(formatParDoError(dofn, len(ret), 3))
	}
	return ret[0], ret[1], ret[2]
}

// ParDo4 inserts a ParDo with 4 outputs into the pipeline.
func ParDo4(s Scope, dofn any, col PCollection, opts ...Option) (PCollection, PCollection, PCollection, PCollection) {
	ret := MustN(TryParDo(s, dofn, col, opts...))
	if len(ret) != 4 {
		panic(formatParDoError(dofn, len(ret), 4))
	}
	return ret[0], ret[1], ret[2], ret[3]
}

// ParDo5 inserts a ParDo with 5 outputs into the pipeline.
func ParDo5(s Scope, dofn any, col PCollection, opts ...Option) (PCollection, PCollection, PCollection, PCollection, PCollection) {
	ret := MustN(TryParDo(s, dofn, col, opts...))
	if len(ret) != 5 {
		panic(formatParDoError(dofn, len(ret), 5))
	}
	return ret[0], ret[1], ret[2], ret[3], ret[4]
}

// ParDo6 inserts a ParDo with 6 outputs into the pipeline.
func ParDo6(s Scope, dofn any, col PCollection, opts ...Option) (PCollection, PCollection, PCollection, PCollection, PCollection, PCollection) {
	ret := MustN(TryParDo(s, dofn, col, opts...))
	if len(ret) != 6 {
		panic(formatParDoError(dofn, len(ret), 6))
	}
	return ret[0], ret[1], ret[2], ret[3], ret[4], ret[5]
}

// ParDo7 inserts a ParDo with 7 outputs into the pipeline.
func ParDo7(s Scope, dofn any, col PCollection, opts ...Option) (PCollection, PCollection, PCollection, PCollection, PCollection, PCollection, PCollection) {
	ret := MustN(TryParDo(s, dofn, col, opts...))
	if len(ret) != 7 {
		panic(formatParDoError(dofn, len(ret), 7))

View on GitHub (pinned to 12126d8942)

Solutions

Update the DoFn's ProcessElement to emit exactly 5 outputs.
Or select the ParDoN helper matching the DoFn's real output count.
Prefer TryParDo + MustN when the count is dynamic, to get a returned error instead of a panic.

Example fix

// before: 4-output DoFn used with ParDo5
func (f *fn) ProcessElement(e string) (string, string, string, string) { ... }
beam.ParDo5(s, &fn{}, col)

// after: emit a fifth output
func (f *fn) ProcessElement(e string) (string, string, string, string, string) { ... }
beam.ParDo5(s, &fn{}, col)

Defensive patterns

Strategy: validation

Validate before calling

if outputsOfMyDoFn != 5 { log.Fatalf("expected 5 outputs, got %d", outputsOfMyDoFn) }

Try / catch

defer func() { if r := recover(); r != nil { err = fmt.Errorf("ParDo5 failed: %v", r) } }()

Prevention

Recount emits after every DoFn refactor
Add a unit test constructing the pipeline to catch arity panics in CI
Use TryParDo to surface arity problems as errors

When it happens

Trigger: Calling beam.ParDo5 with a DoFn whose ProcessElement emits fewer or more than 5 outputs.

Common situations: Off-by-one when adding a new output tag to a 4-output DoFn; reusing a generic fan-out DoFn across pipelines with different output counts; copy-paste of ParDo4 call sites upgraded to ParDo5 without touching the DoFn.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/b441748256caf9f8. Report an issue: GitHub.