apache/beam · error

method has too few main inputs

Error message

%v method has too few main inputs

What it means

validateMainInputs also enforces a minimum main-input count (numMainIn, default MainSingle). When ProcessElement declares fewer main inputs than required (e.g. zero or one where two are expected for KV inputs), this error is raised with a detailed top-level message.

Solutions

1. Declare the required number of main input parameters (e.g. k, v for KVs)
2. Ensure numMainIn passed to NewDoFn/AsDoFn options matches the method signature
3. Check that the incoming PCollection type matches what the method expects

Example fix

// before
func (f *MyFn) ProcessElement(ctx context.Context, k string, emit func(int)) error // expects KV
// after
func (f *MyFn) ProcessElement(ctx context.Context, k string, v int, emit func(int)) error

// ensure ProcessElement's main-input count matches the incoming PCollection shape (KV => 2 params)

When it happens

Trigger: Using a DoFn with numMainIn > 1 (e.g. MainKVs) but declaring ProcessElement with a single element parameter; or dropping one of two parameters for a keyed PCollection.

Common situations: Applying a KV-expecting DoFn to key/value side-input patterns; changing PCollection element type from KV to single value without updating ProcessElement.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• method has no main inputs
• Duplicate state key used by and . Ensure that state keys…
• emit parameter in method
• emit parameters expected in method
• error writing state

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/01292ace951518af. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/graph/fn.go:668

// validateMainInputs checks that a method has the given number of main inputs
// and that main inputs are before any side inputs.
func validateMainInputs(fn *Fn, method *funcx.Fn, methodName string, numMainIn mainInputs) error {
	if numMainIn == MainUnknown {
		numMainIn = MainSingle // If unknown, validate for minimum number of inputs.
	}

	// Make sure there are enough inputs (at least numMainIn)
	pos, num, ok := method.Inputs()
	if !ok {
		err := errors.Errorf("%v method has no main inputs", methodName)
		err = errors.SetTopLevelMsgf(err,
			"Method %v in DoFn %v is missing all inputs. A main input is required.",
			methodName, fn.Name())
		return err
	}
	if num < int(numMainIn) {
		err := errors.Errorf("%v method has too few main inputs", methodName)
		err = errors.SetTopLevelMsgf(err,
			"Method %v in DoFn %v does not have enough main inputs. "+
				"%v main inputs were expected, but only %v inputs were found.",
			methodName, fn.Name(), numMainIn, num)
		return err
	}

	// Check that the first input is not an Iter or ReIter (those aren't valid
	// as the first main input).
	first := method.Param[pos].Kind
	if first != funcx.FnValue {
		err := errors.New("first main input parameter must be a value type")
		err = errors.SetTopLevelMsgf(err,
			"Method %v of DoFns should always have the first input be a value type, "+
				"but it has an Iter or ReIter first in DoFn %v.",
			processElementName, fn.Name())
		return errors.WithContextf(err, "method %v", processElementName)
	}

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