What causes "ordering for List dtype is not supported" in pola-rs/polars?

Do comparisons at the expression/Series level: list.eval / list.get(i) to extract a comparable element, or compute a sort key (list.len(), list.sum(), first element)

pola-rs/polars · error

ordering for List dtype is not supported

Error message

ordering for List dtype is not supported

What it means

AnyValue::partial_cmp has no ordering implementation for List values - nested list scalars are explicitly rejected with unimplemented!(). There is no total, cheap, and universally agreed ordering over variable-length lists at the scalar level.

Source

Thrown at crates/polars-core/src/datatypes/any_value.rs:1441

                lt.partial_cmp(rt)
            },
            #[cfg(feature = "dtype-time")]
            (Time(l), Time(r)) => l.partial_cmp(r),
            #[cfg(feature = "dtype-categorical")]
            (Categorical(l_cat, l_map), Categorical(r_cat, r_map)) => unsafe {
                let l_str = l_map.cat_to_str_unchecked(*l_cat);
                let r_str = r_map.cat_to_str_unchecked(*r_cat);
                l_str.partial_cmp(r_str)
            },
            #[cfg(feature = "dtype-categorical")]
            (Enum(l_cat, l_map), Enum(r_cat, r_map)) => {
                if !Arc::ptr_eq(l_map, r_map) {
                    unimplemented!("can't order enums from different FrozenCategories")
                }
                l_cat.partial_cmp(r_cat)
            },
            (List(_), List(_)) => {
                unimplemented!("ordering for List dtype is not supported")
            },
            #[cfg(feature = "dtype-array")]
            (Array(..), Array(..)) => {
                unimplemented!("ordering for Array dtype is not supported")
            },
            #[cfg(feature = "object")]
            (Object(_), Object(_)) => {
                unimplemented!("ordering for Object dtype is not supported")
            },
            #[cfg(feature = "dtype-struct")]
            (StructOwned(_), StructOwned(_))
            | (StructOwned(_), Struct(..))
            | (Struct(..), StructOwned(_))
            | (Struct(..), Struct(..)) => {
                unimplemented!("ordering for Struct dtype is not supported")
            },
            #[cfg(feature = "dtype-decimal")]
            (Decimal(lv, _lp, ls), Decimal(rv, _rp, rs)) => Some(dec128_cmp(*lv, *ls, *rv, *rs)),

View on GitHub (pinned to df599052da)

Solutions

Do comparisons at the expression/Series level: list.eval / list.get(i) to extract a comparable element, or compute a sort key (list.len(), list.sum(), first element)
Compare element-wise after exploding: df.explode("lst") then order the inner column
Serialize to a comparable representation (e.g. sort the list and hash, or cast to String) when a total order is genuinely needed

Example fix

# before
first_is_smaller = df["lst"].get(0) < df["lst"].get(1)  # panics

# after
first_is_smaller = (
    df["lst"].list.first().get(0) < df["lst"].list.first().get(1)
)

Defensive patterns

Strategy: type-guard

Validate before calling

def orderable_dtype(dt) -> bool:
    return not isinstance(dt, (pl.List, pl.Array, pl.Struct)) and dt != pl.Object

Type guard

def orderable_dtype(dt) -> bool:
    return not isinstance(dt, (pl.List, pl.Array, pl.Struct)) and dt != pl.Object

Prevention

Never compare list scalars with < >; derive a scalar key (list.first, list.len, list.sum) instead
Push ordering into expressions: sort by computed list keys rather than list cells

When it happens

Trigger: Any direct ordering comparison of two List AnyValues: df["lst"].get(0) < df["lst"].get(1), sorted() over list scalars, or generic scalar min/max utilities applied to a List column.

Common situations: User code that pulls individual cell values out of a list column and compares them, or frameworks that sort heterogeneous scalars including lists.

Related errors

AI-assisted analysis of pola-rs/polars@df599052da (2026-08-16). Data as JSON: /api/errors/6708852262740436. Report an issue: GitHub.