pola-rs/polars · error

range end index out of range for slice of length

Error message

range end index {end} out of range for slice of length {len}

What it means

slice_index_fail panics when a range's end exceeds the slice length. This mirrors the standard library's slice bounds message and is reached through polars' check_range helper when a caller requests a slice extending past the end of the underlying data.

Solutions

1. Clamp end to the collection length: end.min(len)
2. Pre-check end <= len and return an empty/truncated result instead
3. Use saturating/checked range construction before calling APIs that slice
4. Fix the upstream length source so the requested end reflects actual data size

Example fix

// before
let out = check_range(start..end, ..len);
// after
let end = end.min(len);
let out = check_range(start..end, ..len);

def safe_end(end, len_):
    if end > len_:
        raise IndexError(f"range end {end} out of range for slice of length {len_}")
    return end
# or clamp: end = min(end, len_)

When it happens

Trigger: Slicing via check_range with an end index greater than the collection length, e.g. asking for rows 5..100 on a 50-row frame segment.

Common situations: Hardcoded slice windows computed from stale lengths; pagination that assumes a minimum record count; taking n..end ranges without knowing size.

Related errors

• range start index out of range for slice of length
• slice index starts at
• activate dtype
• addition overflow: +
• can only use timedelta string language with…

AI-assisted analysis of pola-rs/polars@fe841f959e (2026-09-18). Data as JSON: /api/errors/2db30daa8a8deb06. Report an issue: GitHub.

Appendix: source

Thrown at crates/polars-utils/src/range.rs:10

use std::ops;

// Copied from the stdlib.
fn slice_index_fail(start: usize, end: usize, len: usize) -> ! {
    if start > len {
        panic!("range start index {start} out of range for slice of length {len}",)
    }

    if end > len {
        panic!("range end index {end} out of range for slice of length {len}",)
    }

    if start > end {
        panic!("slice index starts at {start} but ends at {end}",)
    }

    // Only reachable if the range was a `RangeInclusive` or a
    // `RangeToInclusive`, with `end == len`.
    panic!("range end index {end} out of range for slice of length {len}",)
}

#[must_use]
pub fn check_range<R>(range: R, bounds: ops::RangeTo<usize>) -> ops::Range<usize>
where
    R: ops::RangeBounds<usize>,
{
    let len = bounds.end;
    let end = match range.end_bound() {

View on GitHub (pinned to fe841f959e)