pathwaycom/pathway · error · TypeError

'topic_names' must be a str or a list of str; got {type(topi

Error message

'topic_names' must be a str or a list of str; got {type(topic_names).__name__}

What it means

The deprecated 'topic_names' alias in pw.io.kafka.read accepts only a string or a list/tuple of strings (of which just the first element is used, with a warning). Passing any other type — an int, dict, set, None-with-other-keys, etc. — raises TypeError naming the offending type.

Source

Thrown at python/pathway/io/kafka/__init__.py:343

            raise ValueError("Missing topic name specification")
        if isinstance(topic_names, str):
            warnings.warn(
                "'topic_names' is deprecated; please use 'topic' instead.",
                DeprecationWarning,
                stacklevel=_stacklevel + 4,
            )
            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(

View on GitHub (pinned to fa2f74a464)

Solutions

1. Migrate to the modern parameter: pw.io.kafka.read(rdkafka_settings, topic='test-topic').
2. If you must keep topic_names, pass a single string, not a collection or other object.
3. Note that even a valid list only uses its first element — migrate to topic= to avoid silently dropping topics.

Example fix

# before
t = pw.io.kafka.read(rdkafka_settings, topic_names=["a", "b"])
# after
t = pw.io.kafka.read(rdkafka_settings, topic="a")

if topic_names is not None and not isinstance(topic_names, (str, list, tuple)):
    raise SystemExit(f"topic_names must be str or list of str, got {type(topic_names).__name__}")

def valid_topic_names(v) -> bool:
    return v is None or isinstance(v, (str, list, tuple))

When it happens

Trigger: pw.io.kafka.read(rdkafka_settings, topic_names=0), topic_names={'t1': 1}, or topic_names=set(['a']). The check happens after the str and (list, tuple) branches, so only non-str non-sequence types reach it.

Common situations: Passing a pandas/numpy object or a config structure where a string was expected; reusing the deprecated alias from very old Pathway code with unusual payload types.

Related errors

• Got unexpected keyword argument 'topic_name'. pw.io.kafka.re
• Missing topic name specification
• 'topic' must be a non-empty string; got an empty list. Kafka
• 'simple_read' does not accept {forbidden!r}. If you need to
• 'read_only_new=True' together with mode='static' is self-con

AI-assisted analysis of pathwaycom/pathway@fa2f74a464 (2026-08-15). Data as JSON: /api/errors/264a2f710a299452. Report an issue: GitHub.