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Title: Apply titlecase mapping in str.title() for uppercase digraphs by changjoon-park · Pull Request #7748 · RustPython/RustPython · GitHub

Open Graph Title: Apply titlecase mapping in str.title() for uppercase digraphs by changjoon-park · Pull Request #7748 · RustPython/RustPython

X Title: Apply titlecase mapping in str.title() for uppercase digraphs by changjoon-park · Pull Request #7748 · RustPython/RustPython

Description: Background PyStr::title() (crates/vm/src/builtins/str.rs:1040) walks each codepoint and chooses one of three actions per word boundary: Lowercase character starting a new word → call to_titlecase() Uppercase / titlecase character starting a new word → push unchanged Cased character continuing a word → call to_lowercase() Branch 2 is the gap. For ASCII letters and most Unicode characters, to_titlecase(c) == c, so pushing unchanged happens to produce the right result. But Latin Extended-B contains digraph triplets (LATIN LETTERS DZ / DŽ / LJ / NJ) where the uppercase and titlecase forms are distinct codepoints: Codepoint Form U+01C4 'DŽ' uppercase DŽ U+01C5 'Dž' titlecase Dž U+01C6 'dž' lowercase dž U+01F1 'DZ' uppercase DZ U+01F2 'Dz' titlecase Dz U+01F3 'dz' lowercase dz For these, c.to_titlecase() returns a different character than c itself, and the Branch 2 fall-through silently produced the wrong output. Reproduction 'DZ'.title() # CPython: 'Dz' Pre-fix: 'DZ' 'DŽ'.title() # CPython: 'Dž' Pre-fix: 'DŽ' 12 cases probed against CPython 3.14.4 covering all 4 digraph families × 3 forms each (uppercase / titlecase / lowercase): Family upper.title() title.title() lower.title() DŽ (U+01C4-01C6) Dž ✓ Dž ✓ Dž ✓ LJ (U+01C7-01C9) Lj ✓ Lj ✓ Lj ✓ NJ (U+01CA-01CC) Nj ✓ Nj ✓ Nj ✓ DZ (U+01F1-01F3) Dz ✓ Dz ✓ Dz ✓ Fix } else if c.is_uppercase() || c.is_titlecase() { if previous_is_cased { title.extend(c.to_lowercase()); } else { title.extend(c.to_titlecase()); // was: title.push_char(c) } previous_is_cased = true; } The lowercase branch (line 1047) already uses c.to_titlecase() — Branch 2 is now symmetric. char::to_titlecase returns the same character when invoked on an ASCII letter or already-titlecase codepoint, so no existing case regresses. Verification CPython 3.14.4 byte-identical across: 12 single-codepoint digraph cases — all 4 families × 3 forms 8 mixed-string cases — digraphs at start, mid-word, as standalone word, consecutive digraphs, multi-word strings ('hello DZ world' → 'Hello Dz World') 14 sibling-method cases — upper, lower, swapcase, capitalize, casefold, istitle on digraphs unchanged New Rust unit cases added to str_title (crates/vm/src/builtins/str.rs:2664) covering U+01F1 → U+01F2 and U+01C4 → U+01C5. cargo test -p rustpython-vm str_title and str_istitle both pass. Regression sweep: test_str, test_codecs, test_format, test_pprint — 484 tests, 0 regressions. No expectedFailure markers in Lib/test/test_str.py are tied to this root cause; this PR has no test unmask. Issue scope note Issue #7527 listed five examples. Probing on current main shows four already pass: Issue example Status on current main 'Ᲊ'.istitle() == False Stale: CPython 3.14.4 itself returns True for this codepoint 'DZ'.title() == 'Dz' Still failing — fixed by this PR '١'.isdigit() == True Already passes 'ౝ'.isidentifier() == True Already passes '㐅'.isnumeric() == True Already passes This PR closes the only remaining actionable case. The Ᲊ.istitle() expected value in the issue body appears to predate a Unicode database update in CPython 3.14 — current CPython behavior matches RustPython. Closes #7527 Summary by CodeRabbit Bug Fixes Corrected title-casing for Unicode characters whose titlecase mapping expands to multiple characters (including Latin Extended‑B digraphs), so title() now produces proper multi-character expansions and consistent title-case output across scripts.

Open Graph Description: Background PyStr::title() (crates/vm/src/builtins/str.rs:1040) walks each codepoint and chooses one of three actions per word boundary: Lowercase character starting a new word → call to_titlecase(...

X Description: Background PyStr::title() (crates/vm/src/builtins/str.rs:1040) walks each codepoint and chooses one of three actions per word boundary: Lowercase character starting a new word → call to_titlecase(...

Opengraph URL: https://github.com/RustPython/RustPython/pull/7748

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