pola-rs/polars · error · TypeError

cannot turn into selector

Error message

cannot turn {qualified_type_name(input)!r} into selector

What it means

parse_into_expression converts Python inputs into polars expressions. When require_selector is set, the input must be a string, Selector, or Expr; anything else (int, dict, arbitrary object) cannot become a selector, so a TypeError is raised with the qualified type name.

Solutions

1. Pass a column name string (converted to F.col) or a pl.selectors.* selector.
2. Wrap a literal explicitly with pl.lit(value) if you intended a literal, or pl.col(name) for a column.
3. Use pl.Expr.meta.as_selector()-compatible expressions only where selectors are allowed.
4. Check the function signature to see whether it expects expressions, selectors, or names.

Example fix

// before
df.select(42)
// after
df.select(pl.col("column_name"))  # or df.select(pl.selectors.by_index(42))

def ensure_selector_input(x):
    import polars as pl
    if isinstance(x, (str, pl.Selector, pl.Expr)):
        return x
    raise TypeError(f"cannot use {type(x).__name__} as selector; use str/Selector/Expr")

def is_selector_input(x) -> bool:
    import polars as pl
    return isinstance(x, (str, pl.Selector, pl.Expr))

try:
    result = df.select(inputs)
except TypeError as e:
    if "into selector" in str(e):
        result = df.select([pl.lit(v).alias(k) for k, v in inputs.items()])
    else:
        raise

When it happens

Trigger: Calling APIs like pl.selectors-consuming methods or functions that route through parse_into_expression(require_selector=True) with a non-selector value, e.g. df.select(42), pl.slice with a non-string, or passing a list of wrong types where selectors are expected.

Common situations: Typos like passing column positions where names are expected, passing dicts/sets instead of column name strings, or mixing literals into selector-only contexts.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• cannot parse input into Polars selector
• cannot turn into selector
• expected a selector; found
• expected one or more `str`, `DataType` or selector; found
• invalid dtype

AI-assisted analysis of pola-rs/polars@fe841f959e (2026-09-18). Data as JSON: /api/errors/7104bf1a6a1db1f6. Report an issue: GitHub.

Appendix: source

Thrown at py-polars/src/polars/_utils/parse/expr.py:64

        If the input is expected to resolve to a literal with a known dtype, pass
        this to the `lit` constructor.
    require_selector
        Require that the input is a valid selector (eg: column name or selector).

    Returns
    -------
    PyExpr
    """
    if isinstance(input, pl.Expr):
        expr = input
        if structify:
            expr = _structify_expression(expr)
    elif isinstance(input, str) and not str_as_lit:
        expr = F.col(input)
    else:
        if require_selector:
            msg = f"cannot turn {qualified_type_name(input)!r} into selector"
            raise TypeError(msg)
        elif isinstance(input, list) and list_as_series:
            expr = F.lit(pl.Series(input), dtype=dtype)
        else:
            expr = F.lit(input, dtype=dtype)

    return expr._pyexpr


def _structify_expression(expr: Expr) -> Expr:
    unaliased_expr = expr.meta.undo_aliases()
    if unaliased_expr.meta.has_multiple_outputs():
        try:
            expr_name = expr.meta.output_name()
        except ComputeError:
            expr = F.struct(expr)
        else:
            expr = F.struct(unaliased_expr).alias(expr_name)
    return expr

View on GitHub (pinned to fe841f959e)