What causes "panic(formatParDoError(dofn, len(ret), 4))" in apache/beam?

Make the DoFn's ProcessElement emit exactly 4 outputs (4 emit fields or 4 return values).

panic(formatParDoError(dofn, len(ret), 4))

Error message

panic(formatParDoError(dofn, len(ret), 4))

What it means

ParDo4 applies a DoFn expected to produce exactly 4 outputs. If TryParDo returns a count other than 4, the library panics via formatParDoError with the DoFn name, actual count, and expected count of 4.

Source

Thrown at sdks/go/pkg/beam/pardo.go:460

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

// ParDo3 inserts a ParDo with 3 outputs into the pipeline.
func ParDo3(s Scope, dofn any, col PCollection, opts ...Option) (PCollection, PCollection, PCollection) {
	ret := MustN(TryParDo(s, dofn, col, opts...))
	if len(ret) != 3 {
		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))

View on GitHub (pinned to 12126d8942)

Solutions

Make the DoFn's ProcessElement emit exactly 4 outputs (4 emit fields or 4 return values).
Or call the ParDoN variant that matches the DoFn's actual output count.
Use TryParDo to get an error instead of a panic when the output count is uncertain.

Example fix

// before: 3-output DoFn used with ParDo4
func (f *fn) ProcessElement(x int) (int, int, int) { ... }
beam.ParDo4(s, &fn{}, col)

// after: add a fourth output
func (f *fn) ProcessElement(x int) (int, int, int, int) { ... }
beam.ParDo4(s, &fn{}, col)

Defensive patterns

Strategy: validation

Validate before calling

// confirm 4 outputs exist before ParDo4
if outputsOfMyDoFn != 4 {
    log.Fatalf("expected 4 outputs, got %d", outputsOfMyDoFn)
}

Try / catch

defer func() { if r := recover(); r != nil { err = fmt.Errorf("ParDo4 failed: %v", r) } }()

Prevention

Match ParDoN number to the DoFn's emit arity exactly
Prefer ParDoN/TryParDo when arity may change
Review panic text from formatParDoError: it names DoFn, actual, expected counts

When it happens

Trigger: Calling beam.ParDo4 (directly or via CoGBK/ReshuffleKV paths) with a DoFn whose ProcessElement does not emit exactly 4 outputs.

Common situations: DoFn was originally single-output and was reused with ParDo4; an emit parameter was removed during refactor; confusion between main output plus tag outputs versus pure emit-based outputs.

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/69d8e97fcb691468. Report an issue: GitHub.