# Polars 1.42.0 — Python Polars 1.42.0

- Product: Polars (https://whatsnew.fyi/product/polars)
- Vendor: Polars
- Date: 2026-06-24
- Version: 1.42.0
- Original notes: https://github.com/pola-rs/polars/releases/tag/py-1.42.0
- Permalink: https://whatsnew.fyi/product/polars/releases/1.42.0

What's New is an index, not a publisher: every entry below links to the vendor's own release notes, which are the authoritative source. Entries are labelled where they are hand-curated sample data, pre-releases, or drawn from a secondary source such as a developer blog.

Reuse: the summaries, labels and curation here are © What's New. Quote freely with attribution and a link back; wholesale republication of the corpus is not permitted — terms: https://whatsnew.fyi/terms. The vendors' own release notes remain their publishers'.

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- **deprecated** — Deprecate casts from string to temporal dtypes
- **changed** — Add bytes-based concurrency control for cloud IO
- **changed** — Stop flushing multiplexer on phase change
- **changed** — Prevent empty chunks from triggering copy in rechunk
- **changed** — Improve performance of struct unnest inside list eval on streaming
- **changed** — Eliminate filters with contradictory predicates
- **added** — Add naive out-of-core spilling to Polars
- **added** — Add experimental strict mode
- **changed** — Optimize non-negative len and null_count comparisons
- **changed** — Optimize len and null_count comparisons
- **added** — Support SQL's implicit JOIN syntax
- **added** — Add Expr.is_sorted
- **added** — Add suggestions in wrong api usage or mismatched column names
- **added** — Add external object_store support for non-native schemes
- **changed** — Skip Delta files and parquet row groups using per-field struct statistics
- **changed** — Improve IR sortedness detection for with_row_index and explode
- **added** — Add DataFrame.is_sorted()
- **added** — Expose fixed-size rolling window expressions in Python visitor
- **added** — Add tiered multi-file parquet metadata resolver
- **changed** — Cache and shuffle DNS for cloud object_store
- **deprecated** — Add a deprecation warning for .explode() without empty_as_null argument

##### ⚠️ Deprecations

- Deprecate casts from string to temporal dtypes (#28056)

##### 🚀 Performance improvements

- Add bytes-based concurrency control for cloud IO (#27924)
- Stop flushing multiplexer on phase change (#28013)
- Prevent empty chunks from triggering copy in rechunk (#27953)
- Improve performance of struct unnest inside list eval on streaming (#27905)
- Eliminate filters with contradictory predicates (#27775)

##### ✨ Enhancements

- Deprecate casts from string to temporal dtypes (#28056)
- Add naive out-of-core spilling to Polars (#27998)
- Add experimental strict mode (#28023)
- Optimize non-negative len and null\_count comparisons (#28002)
- Optimize len and null\_count comparisons (#27727)
- Support SQL's implicit `JOIN` syntax (#27890)
- Add `Expr.is_sorted` (#26708)
- Add suggestions in wrong api usage or mismatched column names (#27954)
- Add external object\_store support for non-native schemes (#27871)
- Skip Delta files and parquet row groups using per-field struct statistics (#27887)
- Improve IR sortedness detection for with\_row\_index and explode (#27875)
- Add `DataFrame.is_sorted()` (#27870)
- Expose fixed-size rolling window expressions in Python visitor (#27108)
- Fix SchemaError using lazy HConcat->Sink (#27770)
- Add tiered multi-file parquet metadata resolver (#27720)
- Cache and shuffle DNS for cloud object\_store (#27659)

##### 🐞 Bug fixes

- Add a deprecation warning for `.explode()` without `empty_as_null` argument (#28040)
- Fix `select(len())` incorrectly returned 0 when using `scan_iceberg` with `pyiceberg` as reader override (#28044)
- Ensure slice expr always observes input order (#28041)
- Fix projection pushdown with row index out of order (#28043)
- Fix cross-join predicates not applied when using `sink_*` functions (#28042)
- Grammar corrections (recommend to → recommend, allows to → allows using) (#28038)
- Ensure optimisations aren't omitted by cached IR nodes (#28004)
- Computed GROUP keys in SQL projections (#27989)
- Handle empty DataFrames in `dt.truncate` and `dt.round` (#26120)
- Address join alias state leakage in SQL CTEs (#27985)
- Support scanning `file://` URIs with percent-encoded paths (#27876)
- Lower SQL `[NOT] IN (subquery)` to semi/anti join (#27888)
- Check for continuity across morsels in AsOf join node (#27461)
- Add test for projection pushdown with hconcat regression (#27937)
- Traverse caches only once in explain (#27939)
- Preserve null placement when collapsing `sort.reverse()` into single sort (#27918)
- Incorrect schema order for hstack in projection pushdown optimizer (#27920)
- Fix duplicated map\_elements warning on streaming engine (#27919)
- Use block\_in\_place\_on for calls which can come from executor thread (#27855)
- Mismatch in max\_threads -> pipeline configuration (#27854)
- Keep maintain\_order on sliced unique (#27852)
- Fix SchemaError using lazy HConcat->Sink (#27770)
- Fix incorrect projection height when selecting only literals (#27825)
- Select with expr slice and len gave incorrect len (#27824)
- Prevent import panic when environment variable set to unexpected value (#27831)

##### 📖 Documentation

- New On-Prem release (#27979)
- Add note on OOM behavior on Kubernetes (#27972)
- Properly render backslashes in CSV `eol_char` docstrings (#27962)
- Add entry for `DataFrame.is_sorted()` (#27913)
- Clarify compat\_level export docs (#27715)
- Add robots.txt disallowing stale docs and added sitemap for better SEO (#27901)
- Update Polars \<> Spark comparison (#27873)
- Bump to patched version (#27851)
- Replace Typeform sign-up URL with new enterprise link (#27838)
- Correct wrong head call (#27848)
- Add Polars On-Prem 0.5.0 release (#27849)
- Correct onprem license helm values (#27847)
- Update connecting Polars Cloud to AWS documentation (#27823)
- Correct cost/connectivity sections (#27813)
- Add sections about egress (#27805)
- Tragic typo in On-Pre

_[Truncated at 4000 characters — full notes: https://github.com/pola-rs/polars/releases/tag/py-1.42.0]_