Apply titlecase mapping in str.title() for uppercase digraphs (RustPython#7748)

changjoon-park · web-flow · commit c2141a765f45 · 2026-05-01T19:45:55.000+09:00
The uppercase/titlecase branch of PyStr::title() pushed characters unchanged when starting a new word, which left Latin Extended-B digraphs (U+01F1 'DZ', U+01C4 'DŽ', etc.) in their uppercase form instead of mapping them to their distinct titlecase counterparts (U+01F2 'Dz', U+01C5 'Dž'). For ASCII letters and characters where to_titlecase is identity this had no effect, hiding the bug for the common case. Mirror the lowercase branch — which already calls to_titlecase() when starting a new word — so both branches symmetrically apply the titlecase mapping. char::to_titlecase is identity for already- titlecase and ASCII-uppercase characters, so existing cases stay correct. Also unmasks test_unicodedata.UnicodeMiscTest.test_bug_4971, which asserts exactly this behavior (`'Ǆ'.title() == 'ǅ'` etc.) and was marked expectedFailure with reason `+ ǅ`. Closes RustPython#7527 (the only example from that issue still failing on 3.14.4; the other four examples already pass on current main).
diff --git a/Lib/test/test_unicodedata.py b/Lib/test/test_unicodedata.py
@@ -359,7 +359,6 @@ def test_bug_5828(self):
             [0]
         )
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON; + ǅ
     def test_bug_4971(self):
         # LETTER DZ WITH CARON: DZ, Dz, dz
         self.assertEqual("\u01c4".title(), "\u01c5")
diff --git a/crates/vm/src/builtins/str.rs b/crates/vm/src/builtins/str.rs
@@ -1053,7 +1053,7 @@ impl PyStr {
                 if previous_is_cased {
                     title.extend(c.to_lowercase());
                 } else {
-                    title.push_char(c);
+                    title.extend(c.to_titlecase());
                 }
                 previous_is_cased = true;
             } else {
@@ -2661,6 +2661,10 @@ mod tests {
             ("Greek Ωppercases ...", "greek ωppercases ..."),
             // spell-checker:disable-next-line
             ("Greek ῼitlecases ...", "greek ῳitlecases ..."),
+            // Latin Extended-B digraphs: uppercase forms map to titlecase forms
+            // (e.g. U+01F1 'DZ' -> U+01F2 'Dz', U+01C4 'DŽ' -> U+01C5 'Dž').
+            ("\u{01F2}", "\u{01F1}"),
+            ("\u{01C5}", "\u{01C4}"),
         ];
         for (title, input) in tests {
             assert_eq!(PyStr::from(input).title().as_str(), Ok(title));

Original file line number	Diff line number	Diff line change
`@@ -359,7 +359,6 @@ def test_bug_5828(self):`
`359`	`359`	`[0]`
`360`	`360`	`)`
`361`	`361`
`362`		`- @unittest.expectedFailure # TODO: RUSTPYTHON; + ǅ`
`363`	`362`	`def test_bug_4971(self):`
`364`	`363`	`# LETTER DZ WITH CARON: DZ, Dz, dz`
`365`	`364`	`self.assertEqual("\u01c4".title(), "\u01c5")`