pathwaycom/pathway · error · ValueError

wrong output universe. Index out of range

Error message

wrong output universe. Index out of range

What it means

When output_universe is given as an integer, _argument_index validates it against the number of arguments of the pandas UDF (len(func_spec.arg_names)). A negative index or one >= the argument count raises ValueError('wrong output universe. Index out of range') before the UDF ever runs.

Source

Thrown at python/pathway/stdlib/utils/pandas_transformer.py:49

        result.append(reduced)

    def _add_tables(first: pw.Table, *tables: pw.Table) -> pw.Table:
        for table in tables:
            first += table.with_universe_of(first)
        return first

    return _add_tables(*result)


def _argument_index(func_spec: FunctionSpec, arg: str | int | None) -> int | None:
    if isinstance(arg, str):
        try:
            return func_spec.arg_names.index(arg)
        except ValueError:
            raise ValueError(f"wrong output universe. No argument of name: {arg}")
    elif isinstance(arg, int):
        if arg < 0 or arg >= len(func_spec.arg_names):
            raise ValueError("wrong output universe. Index out of range")
    return arg


def _pandas_transformer(
    *inputs: pw.Table,
    func_spec: FunctionSpec,
    output_schema: type[schema.Schema],
    output_universe: str | int | None,
) -> pw.Table:
    output_universe_arg_index = _argument_index(func_spec, output_universe)
    func = func_spec.func

    def process_pandas_output(
        result: pd.DataFrame | pd.Series, pandas_input: list[pd.DataFrame] = []
    ):
        if isinstance(result, pd.Series):
            result = pd.DataFrame(result)

View on GitHub (pinned to fa2f74a464)

Solutions

1. Use a valid 0-based index: 0 <= output_universe < number of UDF parameters
2. Prefer the parameter-name form (output_universe='arg_name') so signature order changes cannot break it
3. Count the UDF parameters and fix or drop the output_universe argument

Example fix

# before
@pw.pandas_transformer(output_universe=1)
def f(df: pd.DataFrame) -> pd.DataFrame: ...

# after
@pw.pandas_transformer(output_universe=0)
def f(df: pd.DataFrame) -> pd.DataFrame: ...

import inspect
n = len(inspect.signature(your_udf).parameters)
assert isinstance(output_universe, int) and 0 <= output_universe < n, f'output_universe index must be in [0, {n})'

def output_universe_index_is_valid(func, i: Any) -> bool:
    import inspect
    return isinstance(i, int) and 0 <= i < len(inspect.signature(func).parameters)

When it happens

Trigger: output_universe=1 for a single-argument UDF; output_universe=2 for a two-argument function; using -1 (Python-style negative indexing is rejected); hardcoding an index after removing a UDF parameter.

Common situations: Reusing a decorator config from a multi-input UDF on a simpler one; refactoring the UDF signature (dropping an argument) without updating the index; assuming negative indices are supported.

Related errors

• wrong output universe. No argument of name: {arg}
• resulting universe does not match the universe of the indica
• index of resulting DataFrame must be unique
• direction argument of join should be of type asof_join.Direc
• The behavior argument of join should be of type pathway.temp

AI-assisted analysis of pathwaycom/pathway@fa2f74a464 (2026-08-15). Data as JSON: /api/errors/7a46494a4a758bfd. Report an issue: GitHub.