pathwaycom/pathway · error · ValueError

Unexpected number of columns in table: {len(table._columns)}

Error message

Unexpected number of columns in table: {len(table._columns)}

What it means

Raised by pw.io.pubsub.write at graph-construction time when the input table does not have exactly one column. The connector maps each row to one Pub/Sub message whose payload is that single column's bytes value, so there is no defined mapping for zero, two, or more columns.

Source

Thrown at python/pathway/io/pubsub/__init__.py:127

    ...     "./credentials.json"
    ... )

    If you don't have the topic created yet, you may want to create it first:

    >>> topic_path = publisher.topic_path(project_id, topic_id)  # doctest: +SKIP
    >>> topic = publisher.create_topic(request={"name": topic_path})  # doctest: +SKIP

    After that you can configure the table output with the following code:

    >>> import pathway as pw
    >>> pw.io.pubsub.write(table, publisher, project_id, topic_id)  # doctest: +SKIP

    At last, don't forget to add ``pw.run()`` to run your pipeline.
    """

    columns = list(table._columns.values())
    if len(columns) != 1:
        raise ValueError(
            f"Unexpected number of columns in table: {len(table._columns)}"
        )

    allowed_column_types = (dt.BYTES, dt.ANY)
    if columns[0].dtype not in allowed_column_types:
        raise ValueError("The column should be of the type 'bytes'")

    output_buffer = _OutputBuffer(publisher, project_id, topic_id)
    subscribe(
        table,
        on_change=output_buffer.on_change,
        on_time_end=output_buffer.on_time_end,
        name=name,
        sort_by=sort_by,
    )

View on GitHub (pinned to fa2f74a464)

Solutions

1. Project to a single payload column before writing: pw.io.pubsub.write(t.select(data=t.data), ...).
2. To publish whole rows, serialize each row to bytes in that one column first (e.g. an apply that returns json-encoded bytes).
3. Check the table's column count in tests to catch schema drift.

Example fix

# before
pw.io.pubsub.write(events, publisher, project_id, topic_id)  # events: id, value, ts

# after
payload = events.select(data=events.value)
pw.io.pubsub.write(payload, publisher, project_id, topic_id)

assert len(t.columns) == 1, f"expected 1 payload column, got {len(t.columns)}"
pw.io.pubsub.write(t, publisher, project_id, topic_id)

def single_column_table(t) -> bool:
    return len(t._columns) == 1

When it happens

Trigger: Calling pw.io.pubsub.write(table, publisher, project_id, topic_id) where table has 0 or >=2 columns, e.g. a full input schema (id, value, ...) instead of a projected single-column table.

Common situations: Passing a raw ingested table straight to the writer without a select(); intending to publish JSON of the whole row but not serializing it first; evolving a schema and forgetting to update the projection.

Related errors

• Unexpected number of columns: {len(row)}
• Failed to detect the region of S3 bucket {bucket!r} (HTTP st
• Schema.with_types() argument name has to be an existing colu
• Schema.without() argument {name!r} has to refer to an existi
• schema does not have columns {missing_columns}

AI-assisted analysis of pathwaycom/pathway@fa2f74a464 (2026-08-15). Data as JSON: /api/errors/749b775532514a49. Report an issue: GitHub.