pathwaycom/pathway · error · ValueError

'value' must be specified if 'key' is not None

Error message

'value' must be specified if 'key' is not None

What it means

Raised by MessageQueueOutputFormat.build for format='raw' or 'plaintext' when a 'key' column is given but 'value' is None. In raw/plaintext mode the message payload is exactly one designated column; when key is set you must say which column carries the payload too, because there is no unambiguous default in the presence of a key (the single-column auto-detection is only attempted when key is None).

Source

Thrown at python/pathway/io/_utils.py:571

            for column_name in table._columns:
                cls.add_column_reference_to_extract(
                    table[column_name], columns_to_extract, extracted_field_indices
                )
            table = table.select(*columns_to_extract)
            data_format = api.DataFormat(
                format_type="jsonlines" if format == "json" else "dsv",
                key_field_names=[],
                value_fields=_format_output_value_fields(table),
                delimiter=delimiter,
                schema_registry_settings=maybe_schema_registry_settings(
                    schema_registry_settings
                ),
                subject=subject,
            )
        elif format == "raw" or format == "plaintext":
            value_field_index = None
            if key is not None and value is None:
                raise ValueError("'value' must be specified if 'key' is not None")
            if value is not None:
                value_field_index = cls.add_column_reference_to_extract(
                    value, columns_to_extract, extracted_field_indices
                )
            else:
                column_names = list(table._columns.keys())
                if len(column_names) != 1:
                    raise ValueError(
                        f"'{format}' format without explicit 'value' specification "
                        "can only be used with single-column tables"
                    )
                value = table[column_names[0]]
                value_field_index = cls.add_column_reference_to_extract(
                    value, columns_to_extract, extracted_field_indices
                )

            table = table.select(*columns_to_extract)
            if (

View on GitHub (pinned to fa2f74a464)

Solutions

1. Pass value=pw.this.payload alongside key=pw.this.k.
2. Or drop 'key' if keying is not needed and the table is single-column (auto-detection then applies).

Example fix

# before
pw.io.kafka.write(t, ..., format='raw', key=pw.this.k)

# after
pw.io.kafka.write(t, ..., format='raw', key=pw.this.k, value=pw.this.payload)

if format in ('raw', 'plaintext') and key is not None:
    assert value is not None, "raw/plaintext output with a key requires an explicit value column"

try:
    pw.io.kafka.write(t, ..., format='raw', key=pw.this.k)
except ValueError as e:
    if "'value' must be specified" in str(e):
        pw.io.kafka.write(t, ..., format='raw', key=pw.this.k, value=pw.this.payload)
    else:
        raise

When it happens

Trigger: pw.io.kafka.write(t, ..., format='raw', key=pw.this.k) with no value on a multi-column table; copying key= from a json-format writer into a raw-format one.

Common situations: Keyed raw payloads (e.g. keyed binary blobs) where the author assumed the writer would guess the payload column; gradual addition of key= to an existing raw writer.

Related errors

• '{format}' format without explicit 'value' specification can
• The value column must have one of the following types: {allo
• Failed to install dependencies
• Column {pseudocolumn} has to contain integers only.
• Column {api.TIME_PSEUDOCOLUMN} cannot contain negative times

AI-assisted analysis of pathwaycom/pathway@fa2f74a464 (2026-08-15). Data as JSON: /api/errors/6664c12ec47cf8c1. Report an issue: GitHub.