pola-rs/polars · error · InvalidArgument

given {n_dtypes} dtypes for {n_cols} names

Error message

given {n_dtypes} dtypes for {n_cols} names

What it means

Validation in the legacy `dataframes` strategy: when dtype is passed as a sequence, its length must equal the number of generated column names (cols). A mismatch means dtypes cannot be paired with columns one-to-one. Supply one dtype per column or a single dtype.

Source

Thrown at py-polars/src/polars/testing/parametric/strategies/legacy.py:102

    ...     assert_something(df)
    """
    # create/assign named columns
    if cols is None:
        cols = st.integers(min_value=min_cols, max_value=max_cols).example()
    if isinstance(cols, int):
        names: Sequence[str] = [f"col{n}" for n in range(cols)]
    else:
        names = cols
    n_cols = len(names)

    if dtype is None:
        dtypes: Sequence[PolarsDataType | None] = [None] * n_cols
    elif is_polars_dtype(dtype):
        dtypes = [dtype] * n_cols
    elif isinstance(dtype, Sequence):
        if (n_dtypes := len(dtype)) != n_cols:
            msg = f"given {n_dtypes} dtypes for {n_cols} names"
            raise InvalidArgument(msg)
        dtypes = dtype
    else:
        msg = f"{dtype!r} is not a valid polars datatype"
        raise InvalidArgument(msg)

    # init list of named/typed columns
    return [
        column(name=nm, dtype=tp, unique=unique)
        for nm, tp in zip(names, dtypes, strict=True)
    ]


@deprecated("`create_list_strategy` is deprecated; use `lists` instead.")
def create_list_strategy(
    inner_dtype: PolarsDataType | None = None,
    *,
    select_from: Sequence[Any] | None = None,
    size: int | None = None,

View on GitHub (pinned to 9b5d73fd00)

Solutions

1. Pass the same number of dtypes as column names (or omit dtypes to let them be inferred).

When it happens

Trigger: Thrown at py-polars/src/polars/testing/parametric/strategies/legacy.py:102 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of pola-rs/polars@9b5d73fd00 (2026-08-19). Data as JSON: /api/errors/6034696e4a8de8ed. Report an issue: GitHub.