pola-rs/polars · error · ValueError

`n_rows` cannot be used with `use_pyarrow=True` and `memory_

Error message

`n_rows` cannot be used with `use_pyarrow=True` and `memory_map=False`

What it means

Error "`n_rows` cannot be used with `use_pyarrow=True` and `memory_map=False`" thrown in pola-rs/polars.

Source

Thrown at py-polars/src/polars/io/ipc/functions.py:162

            storage_options=storage_options,
            row_index_name=row_index_name,
            row_index_offset=row_index_offset,
            rechunk=rechunk,
        )

        if columns:
            if isinstance(columns[0], int):
                lf = lf.select(F.nth(columns))  # type: ignore[arg-type]
            else:
                lf = lf.select(columns)

        df = lf._collect_eager()

        return df

    if use_pyarrow and n_rows and not memory_map:
        msg = "`n_rows` cannot be used with `use_pyarrow=True` and `memory_map=False`"
        raise ValueError(msg)

    with prepare_file_arg(
        source, use_pyarrow=use_pyarrow, storage_options=storage_options
    ) as data:
        if use_pyarrow:
            pyarrow_ipc = import_optional(
                "pyarrow.ipc",
                err_prefix="",
                err_suffix="is required when using 'read_ipc(..., use_pyarrow=True)'",
            )

            if columns is not None and is_non_empty_sequence_of(columns, str):
                initial_pos: Any = None

                if hasattr(data, "tell") and callable(data.tell):
                    initial_pos = data.tell()

                with pyarrow_ipc.open_file(data) as ipc_f:

View on GitHub (pinned to df599052da)

When it happens

Trigger: Thrown at py-polars/src/polars/io/ipc/functions.py:162 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of pola-rs/polars@df599052da (2026-08-16). Data as JSON: /api/errors/02519255ea4c52da. Report an issue: GitHub.