gh-156518: Return the simple case mappings defined by the UCD

Misc/NEWS.d/next/C_API/2026-08-28-14-11-45.gh-issue-156518.0qDYQr.rst

@@ -0,0 +1,6 @@
+:c:func:`Py_UNICODE_TOUPPER`, :c:func:`Py_UNICODE_TOLOWER` and
+:c:func:`Py_UNICODE_TOTITLE` now return the simple case mapping defined by the
+Unicode database instead of the first character of the full case mapping.  The
+simple uppercase of ``'ß'`` (U+00DF) is now ``'ß'`` instead of ``'S'``, and the
+simple uppercase of ``'ᾀ'`` (U+1F80) is ``'ᾈ'`` (U+1F88) instead of ``'Ἀ'``
+(U+1F08).

Modules/_sre/sre.c

@@ -595,8 +595,12 @@ static int
 _sre_unicode_iscased_impl(PyObject *module, int character)
 /*[clinic end generated code: output=9c5ddee0dc2bc258 input=51e42c3b8dddb78e]*/
 {
-    unsigned int ch = (unsigned int)character;
-    return ch != sre_lower_unicode(ch) || ch != sre_upper_unicode(ch);
+    Py_UCS4 ch = (Py_UCS4)character;
+    Py_UCS4 mapped[3];
+    /* A cased character can have no simple case mapping, as LATIN SMALL
+       LETTER SHARP S whose uppercase is "SS". */
+    return _PyUnicode_ToUpperFull(ch, mapped) != 1 || mapped[0] != ch ||
+           _PyUnicode_ToLowerFull(ch, mapped) != 1 || mapped[0] != ch;
 }
 
 /*[clinic input]

Objects/unicodectype.c

@@ -64,8 +64,12 @@ Py_UCS4 _PyUnicode_ToTitlecase(Py_UCS4 ch)
 {
     const _PyUnicode_TypeRecord *ctype = gettyperecord(ch);
 
-    if (ctype->flags & EXTENDED_CASE_MASK)
+    if (ctype->flags & EXTENDED_CASE_MASK) {
+        /* No character with a longer full titlecase has a simple one. */
+        if ((ctype->title >> 24) != 1)
+            return ch;
         return _PyUnicode_ExtendedCase[ctype->title & 0xFFFF];
+    }
     return ch + ctype->title;
 }
 
@@ -182,8 +186,25 @@ Py_UCS4 _PyUnicode_ToUppercase(Py_UCS4 ch)
 {
     const _PyUnicode_TypeRecord *ctype = gettyperecord(ch);
 
-    if (ctype->flags & EXTENDED_CASE_MASK)
+    if (ctype->flags & EXTENDED_CASE_MASK) {
+        if ((ctype->upper >> 24) != 1) {
+            /* Only the full mapping is stored when it is longer than one
+               character (see Tools/unicode/makeunicodedata.py).  Most such
+               characters have no simple uppercase, but the Greek letters
+               with ypogegrammeni have the one with prosgegrammeni.  All of
+               them are Unicode 1.1. */
+            if (ch >= 0x1F80 && ch <= 0x1FA7 && !(ch & 8)) {
+                return ch + 8;
+            }
+            switch (ch) {
+            case 0x1FB3: return 0x1FBC;  /* ALPHA WITH YPOGEGRAMMENI */
+            case 0x1FC3: return 0x1FCC;  /* ETA WITH YPOGEGRAMMENI */
+            case 0x1FF3: return 0x1FFC;  /* OMEGA WITH YPOGEGRAMMENI */
+            }
+            return ch;
+        }
         return _PyUnicode_ExtendedCase[ctype->upper & 0xFFFF];
+    }
     return ch + ctype->upper;
 }
 
@@ -194,8 +215,14 @@ Py_UCS4 _PyUnicode_ToLowercase(Py_UCS4 ch)
 {
     const _PyUnicode_TypeRecord *ctype = gettyperecord(ch);
 
-    if (ctype->flags & EXTENDED_CASE_MASK)
+    if (ctype->flags & EXTENDED_CASE_MASK) {
+        if ((ctype->lower >> 24) != 1) {
+            /* LATIN CAPITAL LETTER I WITH DOT ABOVE is the only character
+               with a longer full lowercase and a simple one. */
+            return ch == 0x0130 ? 0x0069 : ch;
+        }
         return _PyUnicode_ExtendedCase[ctype->lower & 0xFFFF];
+    }
     return ch + ctype->lower;
 }