apache/beam · error

panic(formatParDoError(dofn, len(ret), 1))

Error message

panic(formatParDoError(dofn, len(ret), 1))

What it means

ParDo applies a ParDo transform that must produce exactly one output PCollection. If TryParDo returns a number of PCollections other than 1, it panics with a formatted ParDo error naming the dofn, actual count, and expected count of 1.

Solutions

1. Ensure the DoFn's ProcessElement returns exactly one output value (plus optional error per Beam Go conventions)
2. Use ParDo0 for no-output DoFns or ParDo2/ParDo3... for multi-output DoFns
3. Read formatParDoError's message to see the actual vs expected count and adjust accordingly

Example fix

// before
beam.ParDo(s, func(x int, emit func(int), emit2 func(int)) {...}, col) // 2 outputs
// after
o1, o2 := beam.ParDo2(s, func(x int, emit func(int), emit2 func(int)) {...}, col)

// A ParDo DoFn must produce exactly one output per element

func isSingleOutputDoFn(fn any) bool {
    t := reflect.TypeOf(fn)
    for t.Kind() == reflect.Ptr {
        t = t.Elem()
    }
    m, ok := t.MethodByName("ProcessElement")
    if !ok { return false }
    // heuristic: exactly one emit param or one non-error return value
    numEmit := 0
    for i := 0; i < m.Type.NumIn(); i++ {
        if m.Type.In(i).Kind() == reflect.Func { numEmit++ }
    }
    return numEmit == 1 || (m.Type.NumOut() == 1)
}

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

When it happens

Trigger: Calling beam.ParDo with a DoFn that returns zero values (side-effect only) or multiple outputs (e.g. (string, int) or ProcessElement with multiple emitters).

Common situations: Using a multi-output DoFn (written for ParDo2/ParDo3) with ParDo, or a sink-style DoFn with no returns passed to ParDo after a refactor.

Related errors

• panic(formatParDoError(dofn, len(ret), 0))
• panic(formatParDoError(dofn, len(ret), 2))
• CreateInitialRestriction has unexpected number of parameters
• CreateTracker has unexpected number of parameters
• CreateWatermarkEstimator fn

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/249e579f0b1f5f95. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/pardo.go:431

// one between a producer ParDo and a consumer ParDo), the PCollection itself
// is "fused away" and won't ever be written to disk, saving all the I/O and
// space expense of constructing it.
//
// When Beam runners apply fusion optimization, it is essentially "free" to
// write ParDo operations in a very modular, composable style, each ParDo
// operation doing one clear task, and stringing together sequences of ParDo
// operations to get the desired overall effect. Such programs can be easier to
// understand, easier to unit-test, easier to extend and evolve, and easier to
// reuse in new programs. The predefined library of PTransforms that come with
// Beam makes heavy use of this modular, composable style, trusting to the
// runner to "flatten out" all the compositions into highly optimized stages.
//
// See https://beam.apache.org/documentation/programming-guide/#pardo
// for the web documentation for ParDo
func ParDo(s Scope, dofn any, col PCollection, opts ...Option) PCollection {
	ret := MustN(TryParDo(s, dofn, col, opts...))
	if len(ret) != 1 {
		panic(formatParDoError(dofn, len(ret), 1))
	}
	return ret[0]
}

// TODO(herohde) 6/1/2017: add windowing aspects to above documentation.

// ParDo2 inserts a ParDo with 2 outputs into the pipeline.
func ParDo2(s Scope, dofn any, col PCollection, opts ...Option) (PCollection, PCollection) {
	ret := MustN(TryParDo(s, dofn, col, opts...))
	if len(ret) != 2 {
		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...))

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