pola-rs/polars · error · TypeError

expected a selector; found {selector!r} instead.

Error message

expected a selector; found {selector!r} instead.

What it means

polars.selectors.expand_selector requires an actual selector: with strict=True (default) the value must pass is_selector (a polars Selector). With strict=False it may be any column-selection expression (meta.is_column_selection(allow_aliasing=False)). Strings, plain columns like pl.col('a') under strict mode, and computed expressions all fail.

Source

Thrown at py-polars/src/polars/selectors.py:185

    Allow for non-strict selection expressions (such as those
    including use of an `.exclude()` constraint) to be expanded:

    >>> cs.expand_selector(schema, cs.numeric().exclude("id"), strict=False)
    ('count', 'value')
    """
    if isinstance(target, Mapping):
        from polars.dataframe import DataFrame

        target = DataFrame(schema=target)

    if not (
        is_selector(selector)
        if strict
        else selector.meta.is_column_selection(allow_aliasing=False)
    ):
        msg = f"expected a selector; found {selector!r} instead."
        raise TypeError(msg)

    return tuple(target.select(selector).collect_schema())


# TODO: Don't use this as it collects a schema (can be very expensive for LazyFrame).
#  This should move to IR conversion / Rust.
def _expand_selectors(frame: DataFrame | LazyFrame, *items: Any) -> builtins.list[str]:
    """
    Internal function that expands any selectors to column names in the given input.

    Non-selector values are left as-is.

    Examples
    --------
    >>> from polars.selectors import _expand_selectors
    >>> import polars.selectors as cs
    >>> df = pl.DataFrame(
    ...     {

View on GitHub (pinned to df599052da)

Solutions

1. Pass a real selector: expand_selector(df, cs.by_name('a'))
2. For plain column expressions allow non-strict mode: expand_selector(df, pl.col('a'), strict=False)
3. Replace pl.all() with cs.all() when a Selector type is required

Example fix

# before
expand_selector(df, pl.col('a'))  # strict -> TypeError

# after
expand_selector(df, cs.by_name('a'))
# or, for a plain column selection expr:
expand_selector(df, pl.col('a'), strict=False)

from polars.selectors import is_selector
assert is_selector(sel), f'got {sel!r}; wrap with cs.by_name()/cs.all()'

from polars.selectors import is_selector

def requires_selector(sel) -> bool:
    return is_selector(sel)

When it happens

Trigger: expand_selector(df, 'a'); expand_selector(df, pl.col('a')) with default strict=True; expand_selector(df, pl.col('a') + 1) in any mode; passing pl.all() (an Expr, not a Selector) where cs.all() is required.

Common situations: Internal/shared helper code that accepts 'selectors or column-like things' and forwards unconverted values; pandas-influenced code passing strings; confusing pl.all() with cs.all().

Related errors

• expected one or more `str`, `DataType` or selector; found {i
• cannot parse input {input_type} into Polars selector{input_d
• unsupported operand type(s) for op: ('Selector' + 'Selector'
• unsupported operand type(s) for op: ('Expr' - 'Selector')
• `new` argument is required if `old` argument is not a Mappin

AI-assisted analysis of pola-rs/polars@df599052da (2026-08-16). Data as JSON: /api/errors/6dbf322aa3533374. Report an issue: GitHub.