pathwaycom/pathway · error · ValueError

'topic' must be a non-empty string; got an empty list. Kafka

Error message

'topic' must be a non-empty string; got an empty list. Kafka does not allow empty topic names.

What it means

pw.io.kafka.read tolerates a list-valued 'topic' argument for backward compatibility (using the first element with a DeprecationWarning), but an empty list carries no topic at all and is rejected: Kafka does not permit empty topic names, and Pathway raises ValueError before the consumer would fail less clearly.

Source

Thrown at python/pathway/io/kafka/__init__.py:349

            )
            topic = topic_names
        elif isinstance(topic_names, (list, tuple)):
            warnings.warn(
                "'topic_names' is deprecated; please use 'topic' instead. "
                "Only the first element of the provided list is used as "
                "the topic name.",
                DeprecationWarning,
                stacklevel=_stacklevel + 4,
            )
            topic = topic_names[0]
        else:
            raise TypeError(
                f"'topic_names' must be a str or a list of str; got "
                f"{type(topic_names).__name__}"
            )
    if isinstance(topic, list):
        if not topic:
            raise ValueError(
                "'topic' must be a non-empty string; got an empty list. "
                "Kafka does not allow empty topic names."
            )
        warnings.warn(
            "'topic' should be a str, not list. First element will be used.",
            DeprecationWarning,
            stacklevel=_stacklevel + 4,
        )
        topic = topic[0]

    if not isinstance(topic, str) or not topic:
        raise ValueError(
            f"'topic' must be a non-empty string; got {topic!r}. "
            "Kafka does not allow empty topic names."
        )

    check_deprecated_kwargs(kwargs, ["topic_names"], stacklevel=_stacklevel + 4)

View on GitHub (pinned to fa2f74a464)

Solutions

1. Pass a non-empty string: topic='test-topic'.
2. If topics come from config, validate non-emptiness before calling read and fail with your own actionable message.
3. Migrate away from list-valued topic entirely — only the first element was ever used.

Example fix

# before
topics = []  # populated dynamically, ended up empty
t = pw.io.kafka.read(rdkafka_settings, topic=topics)
# after
topics = ["test-topic"]
t = pw.io.kafka.read(rdkafka_settings, topic=topics[0])

if isinstance(topic, list):
    if not topic:
        raise SystemExit("topic list is empty; provide at least one topic name")
    topic = topic[0]

t = pw.io.kafka.read(rdkafka_settings, topic=topic)

def usable_topic(v) -> bool:
    return (isinstance(v, str) and bool(v)) or (isinstance(v, list) and len(v) > 0)

When it happens

Trigger: pw.io.kafka.read(rdkafka_settings, topic=[]) — an empty list is detected before the first-element fallback runs. Contrast with topic=['a'] which only warns, and topic='' which hits the string check instead.

Common situations: Building the topic list dynamically (e.g. from an environment variable or subscription registry) and passing it through when it happens to be empty; refactoring code that used to pass a list of topics and now passes zero.

Related errors

• Failed to install dependencies
• Column {pseudocolumn} has to contain integers only.
• Column {api.TIME_PSEUDOCOLUMN} cannot contain negative times
• You cannot use {dep.to_column_expression()} in this reduce s
• join condition should be of form <left_table>.<column> == <r

AI-assisted analysis of pathwaycom/pathway@fa2f74a464 (2026-08-15). Data as JSON: /api/errors/3a83bdce195f4491. Report an issue: GitHub.