Fix title() and capitalize() by joshuamegnauth54 · Pull Request #7717 · RustPython/RustPython

joshuamegnauth54 · 2026-04-28T17:51:02Z

I fixed title() and capitalize() as well as enabled their respective tests. I fixed both by following CPython's logic closely, including implementing a workaround for sigma.

Besides that, I removed an unneeded dependency (unicode-casing) by using icu4x instead. This helps with consistency and removes outdated Unicode tables from the final RustPython binary.

Summary by CodeRabbit

Bug Fixes
- Improved Unicode string case behavior (capitalize, title, istitle), including correct handling of Greek sigma, Turkish dotted I, German ß, combining marks, emoji-leading text, and preservation of invalid bytes.
Chores
- Switched to ICU-based Unicode libraries and added a formatting/helper dependency.
Refactor
- Reworked titlecasing and ASCII-bytes logic for clarity and reuse.
Tests
- Enabled and expanded Unicode title/capitalize tests covering multiple edge cases.

coderabbitai · 2026-04-28T17:51:11Z

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Run ID: 80cb2aad-e0ff-4825-b958-db496724819d

📥 Commits

Reviewing files that changed from the base of the PR and between cbce58c and ce75f32.

⛔ Files ignored due to path filters (2)

Cargo.lock is excluded by !**/*.lock
Lib/test/test_str.py is excluded by !Lib/**

📒 Files selected for processing (6)

Cargo.toml
crates/vm/Cargo.toml
crates/vm/src/builtins/str.rs
crates/vm/src/bytes_inner.rs
crates/vm/src/utils.rs
extra_tests/snippets/builtin_str.py

✅ Files skipped from review due to trivial changes (1)

extra_tests/snippets/builtin_str.py

🚧 Files skipped from review as they are similar to previous changes (5)

crates/vm/src/utils.rs
Cargo.toml
crates/vm/Cargo.toml
crates/vm/src/builtins/str.rs
crates/vm/src/bytes_inner.rs

📝 Walkthrough

Walkthrough

Replaces unicode-casing with ICU crates and writeable; rewrites PyStr casing methods (capitalize, title, istitle) to use ICU titlecasing and Unicode property checks; extracts an ASCII title helper for bytes; adds a Vec-backed fmt::Write helper; enables/extends related string tests.

Changes

Unicode casing & string title/capitalize overhaul

Layer / File(s)	Summary
Dependencies `Cargo.toml`, `crates/vm/Cargo.toml`	`workspace` and `crates/vm` manifests updated: removed `unicode-casing`, added `icu_casemap`, `icu_locale`, `icu_normalizer`, `icu_properties`, and `writeable`; reordered related entries.
Imports / Traits `crates/vm/src/builtins/str.rs`	Updated casing-related imports to use ICU casemap/locale, Unicode property groups, and `Writeable`; removed prior `unicode_casing` imports.
Infrastructure `crates/vm/src/utils.rs`	Added `pub(crate) struct VecFmtWriter(pub Vec<u8>)` and `impl fmt::Write` to allow writing UTF-8 text into a `Vec<u8>`.
Bytes Helpers `crates/vm/src/bytes_inner.rs`	Introduced `pub(crate) fn title_ascii(bytes: &[u8]) -> Vec<u8>`; `PyBytesInner::title` now delegates to this ASCII-only helper.
Capitalize (UTF-8) `crates/vm/src/builtins/str.rs`	`PyStr::capitalize` UTF-8 path uses `titlecase_first` (ICU via Writeable) then `lowercase_or_sigma` for remaining characters.
Capitalize (WTF-8) `crates/vm/src/builtins/str.rs`	`PyStr::capitalize` WTF-8 path processes valid UTF-8 chunks with ICU, preserves invalid bytes between chunks.
Title dispatch `crates/vm/src/builtins/str.rs`	`PyStr::title` dispatches: ASCII -> `title_ascii`, UTF-8 -> `titlecase_string`, WTF-8 -> chunked `titlecase_string` preserving invalid bytes.
istitle update `crates/vm/src/builtins/str.rs`	`PyStr::istitle` now checks `GeneralCategoryGroup::TitlecaseLetter` (and uppercase) for titlecase eligibility.
Helpers `crates/vm/src/builtins/str.rs`	Added `titlecase_first`, `titlecase_string`, `lowercase_or_sigma`, and `handle_capital_sigma` implementing ICU-backed title behavior and sigma selection.
Tests `extra_tests/snippets/builtin_str.py`	Enabled `"Ǳ".title() == "ǲ"` and added `str.capitalize()` assertions for `\"ßello\"`, `\"İstanbul\"`, `\"a\u0301bc\"`, `\"ΣΙΓΜΑ\"`, `\"😀hello\"`, and `\"élan\"`.

Sequence Diagram(s)

sequenceDiagram
  participant Caller
  participant PyStr
  participant ICU
  participant Props
  participant Bytes
  Caller->>PyStr: call .capitalize() / .title() / .istitle()
  PyStr->>ICU: titlecase_string / titlecase_first (language-aware mapping)
  PyStr->>Props: query GeneralCategory / TitlecaseLetter (sigma/context checks)
  ICU-->>PyStr: mapped titlecase segments (Writeable output)
  PyStr->>PyStr: lowercase_or_sigma / handle_capital_sigma adjustments
  PyStr->>Bytes: title_ascii for ASCII-only fast path or bytes chunks
  Bytes-->>PyStr: ASCII-chunk results
  PyStr-->>Caller: return final cased string (WTF-8: preserve invalid bytes)

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~60 minutes

Possibly related PRs

RustPython/RustPython#7559: Modifies Unicode-aware casing in crates/vm/src/builtins/str.rs, related to casing logic changes.
RustPython/RustPython#7748: Adjusts titlecasing behavior in crates/vm/src/builtins/str.rs.
RustPython/RustPython#7555: Prior ICU migration work touching workspace/crate dependencies and casing logic.

Suggested reviewers

ShaharNaveh
youknowone

Poem

A rabbit found a casing clue,
Swapped crates and stitched bytes true,
Sigma learned its contextual tune,
Vecs wrote letters by the moon,
Hooray — the strings now wake anew! 🐇✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title directly reflects the main changes: fixes to str.title() and str.capitalize() implementations with ICU-based titlecasing integration.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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github-actions · 2026-05-02T00:54:04Z

📦 Library Dependencies

The following Lib/ modules were modified. Here are their dependencies:

[x] test: cpython/Lib/test/test_str.py (TODO: 8)
[x] test: cpython/Lib/test/test_fstring.py (TODO: 19)
[x] test: cpython/Lib/test/test_string_literals.py (TODO: 4)

dependencies:

dependent tests: (no tests depend on str)

Legend:

[+] path exists in CPython
[x] up-to-date, [ ] outdated

ShaharNaveh

Overall LGTM!

ty:)

ShaharNaveh · 2026-05-04T07:01:05Z

@joshuamegnauth54 can you please fix the merge conflicts?

joshuamegnauth54 · 2026-05-04T17:12:14Z

Fixed and also I fixed a small clippy lint that I introduced. I'm not sure why it didn't trigger earlier, but one of my helpers should have been pub(crate) instead of just pub.

ShaharNaveh · 2026-05-04T17:38:44Z

Fixed and also I fixed a small clippy lint that I introduced. I'm not sure why it didn't trigger earlier, but one of my helpers should have been pub(crate) instead of just pub.

That's because we have added a new lint for that at #7762 :)

ShaharNaveh

LGTM!
tysm:)

youknowone · 2026-05-06T11:40:34Z

I am sorry, I merged another PR first and it made conflict. Could you resolve conflict? I will merge it immediately

joshuamegnauth54 · 2026-05-06T18:23:26Z

Fixed and force pushed. 😁

ShaharNaveh

lgtm!
ty:)

`icu_casemap` is consistently maintained, official, and tracks the latest Unicode versions. RustPython is also using other `icu4x` crates, so using `icu_casemap` is more consistent. As with islower and isupper, tracking the latest Unicode version is important because character definitions shift over time which causes discrepancies between RustPython and CPython. This commit fixes title().

I dropped unicode-casing because it's cleaner to use icu4x for everything. `icu4x` will also stay up to date whereas unicode-casing will need to be periodically updated with new Unicode tables. Dropping unicode-casing also removes some binary bloat due to the tables. `capitalize()` mimics CPython behavior more closely now as well. Notably, I implemented CPython's sigma edge case handler.

joshuamegnauth54 changed the title ~~Titlecase fixes~~ Fix title() and capitalize() May 2, 2026

joshuamegnauth54 marked this pull request as ready for review May 2, 2026 00:53

joshuamegnauth54 force-pushed the titlecase-fixes branch from c533354 to 873c42b Compare May 2, 2026 00:53

ShaharNaveh requested changes May 2, 2026

View reviewed changes

Comment thread Lib/test/string_tests.py

joshuamegnauth54 force-pushed the titlecase-fixes branch from 873c42b to 109a290 Compare May 3, 2026 22:18

youknowone enabled auto-merge (squash) May 4, 2026 11:47

auto-merge was automatically disabled May 4, 2026 17:10
Head branch was pushed to by a user without write access

joshuamegnauth54 force-pushed the titlecase-fixes branch from 109a290 to cbce58c Compare May 4, 2026 17:10

ShaharNaveh approved these changes May 4, 2026

View reviewed changes

joshuamegnauth54 mentioned this pull request May 4, 2026

Use icu4x for casefold() #7780

Draft

youknowone enabled auto-merge (squash) May 6, 2026 11:39

auto-merge was automatically disabled May 6, 2026 18:19
Head branch was pushed to by a user without write access

joshuamegnauth54 force-pushed the titlecase-fixes branch from cbce58c to 114fc53 Compare May 6, 2026 18:19

ShaharNaveh approved these changes May 7, 2026

View reviewed changes

youknowone enabled auto-merge (squash) May 9, 2026 15:32

joshuamegnauth54 added 4 commits May 10, 2026 00:47

PyBytes.title should be ASCII-only.

9d8d34e

Match CPython's title() exactly

ce75f32

youknowone force-pushed the titlecase-fixes branch from 114fc53 to ce75f32 Compare May 9, 2026 16:10

youknowone merged commit 108461f into RustPython:main May 9, 2026
26 checks passed

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Walkthrough

Changes

Sequence Diagram(s)

Estimated code review effort

Possibly related PRs

Suggested reviewers

Poem

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📦 Library Dependencies

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joshuamegnauth54 commented Apr 28, 2026 •

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