pathwaycom/pathway · error · ValueError

'value' and format='{format}' cannot be set at the same time

Error message

'value' and format='{format}' cannot be set at the same time

What it means

Raised by MessageQueueOutputFormat.build when 'value' is passed together with format='json' or format='dsv'. In those formats the writer serializes the whole table row (all columns) into the payload, so designating a single value column is contradictory; 'value' is only meaningful for 'raw'/'plaintext', which send exactly one column's bytes.

Source

Thrown at python/pathway/io/_utils.py:538

                        "headers (pathway_time / pathway_diff) and cannot be "
                        "used as a user header name. Alias the column to "
                        "another name with `table.select(<new_name>=...)`."
                    )
                if header.name in header_fields:
                    raise ValueError(
                        f"Duplicate header name {header.name!r}: two columns "
                        "produce a header with the same name. Alias one of "
                        "them to a different name (e.g. via `table.select(...)`) "
                        "to keep both as separate Kafka headers."
                    )
                header_fields[header.name] = cls.add_column_reference_to_extract(
                    header, columns_to_extract, extracted_field_indices
                )

        # Format-dependent parts: handle json and dsv separately
        if format == "json" or format == "dsv":
            if value is not None:
                raise ValueError(
                    f"'value' and format='{format}' cannot be set at the same time"
                )
            if format == "json":
                reserved = {"time", "diff"}
                conflicting = reserved.intersection(table._columns.keys())
                if conflicting:
                    raise ValueError(
                        f"The table has columns {sorted(conflicting)} which "
                        f"clash with the reserved JSON fields written by the "
                        f"connector ('time', 'diff'). Rename or drop the "
                        f"conflicting column(s) before writing in 'json' "
                        f"format, otherwise the output JSON would contain "
                        f"duplicate keys."
                    )
            for column_name in table._columns:
                cls.add_column_reference_to_extract(
                    table[column_name], columns_to_extract, extracted_field_indices
                )

View on GitHub (pinned to fa2f74a464)

Solutions

1. Remove the 'value' argument when format='json' or 'dsv' (all columns are serialized automatically).
2. If only some columns should be emitted, table.select() the desired columns first.
3. If you truly want a single column as the raw payload, use format='raw' with value=.

Example fix

# before
pw.io.kafka.write(t, ..., format='json', value=pw.this.payload)

# after
t = t.select(pw.this.payload)  # if only that column is wanted
pw.io.kafka.write(t, ..., format='json')

if format in ('json', 'dsv'):
    assert value is None, "'value' cannot be combined with json/dsv output formats"

try:
    pw.io.kafka.write(t, ..., format=format, value=value)
except ValueError as e:
    if "cannot be set at the same time" in str(e):
        pw.io.kafka.write(t, ..., format=format)
    else:
        raise

When it happens

Trigger: pw.io.kafka.write(t, ..., format='json', value=pw.this.payload); switching a raw writer to json while keeping the value argument; reusing a kwargs dict between raw and json writers.

Common situations: Gradually migrating a raw-payload writer to structured json output and leaving the old value= behind; copy-paste between sink configurations.

Related errors

• Unsupported argument for {data_format!r} format: 'value'
• 'schema_registry_settings' is only meaningful for the 'json'
• Unsupported format: {format}
• The sort_by column {column.name!r} is not part of the data b
• Failed to install dependencies

AI-assisted analysis of pathwaycom/pathway@fa2f74a464 (2026-08-15). Data as JSON: /api/errors/b25a68d49105c2da. Report an issue: GitHub.