gh-126219: Fix crash in tkinter.Tk with non-BMP className on Tcl/Tk 8.x

Tcl 8.x crashes when title-casing a non-BMP character during Tk
initialization, so such a className is now rejected with a ValueError.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/test/test_tkinter/test_misc.py

@@ -10,7 +10,8 @@
 from test.support import os_helper
 from test.test_tkinter.support import setUpModule  # noqa: F401
 from test.test_tkinter.support import (AbstractTkTest, AbstractDefaultRootTest,
-                                       requires_tk, get_tk_patchlevel)
+                                       requires_tk, get_tk_patchlevel,
+                                       tcl_version)
 
 support.requires('gui')
 
@@ -51,6 +52,15 @@ class Button2(tkinter.Button):
         b4 = Button2(f2)
         self.assertEqual(len({str(b), str(b2), str(b3), str(b4)}), 4)
 
+    def test_nonbmp_className(self):
+        # gh-126219: a non-BMP character in className crashed Tcl/Tk 8.x while
+        # title-casing the class name during Tk initialization.
+        if tcl_version >= (9, 0):
+            # Tcl 9 supports non-BMP characters.
+            tkinter.Tk(className='\U0010FFFF').destroy()
+        else:
+            self.assertRaises(ValueError, tkinter.Tk, className='\U0010FFFF')
+
     @requires_tk(8, 6, 6)
     def test_tk_busy(self):
         root = self.root

Misc/NEWS.d/next/Library/2026-06-23-13-27-14.gh-issue-126219.kOfv2g.rst

@@ -0,0 +1,3 @@
+Fixed a crash in :class:`tkinter.Tk` when *className* contains a non-BMP
+character and tkinter is built against Tcl/Tk 8.x.  Such a name is now
+rejected with a :exc:`ValueError`.

Modules/_tkinter.c

@@ -3272,6 +3272,20 @@ _tkinter_create_impl(PyObject *module, const char *screenName,
     CHECK_STRING_LENGTH(className);
     CHECK_STRING_LENGTH(use);
 
+#if TCL_MAJOR_VERSION < 9
+    /* className is title-cased during Tk initialization.  Tcl 8.x does not
+     * support non-BMP characters (encoded as 4-byte UTF-8 sequences) there
+     * and crashes in Tcl_UtfToTitle (see gh-126219).  Reject them up front. */
+    for (const unsigned char *p = (const unsigned char *)className; *p; p++) {
+        if (*p >= 0xF0) {
+            PyErr_SetString(PyExc_ValueError,
+                            "className must not contain non-BMP characters "
+                            "with this version of Tcl/Tk");
+            return NULL;
+        }
+    }
+#endif
+
     return (PyObject *) Tkapp_New(screenName, className,
                                   interactive, wantobjects, wantTk,
                                   sync, use);