gh-161388: [tkinter] Fix crash on raising __str__ method passed to setvar

Author: sobolevn

Lib/test/test_tcl.py

@@ -596,6 +596,17 @@ def test_returning_deeply_nested(self):
                     % support.exceeds_recursion_limit())
         self.assertRaises(RecursionError, interp.getvar, 'x')
 
+    def test_setvar_evil(self):
+        # gh-161388
+        class StrRaises:
+            def __str__(self):
+                raise RuntimeError("setvar evil")
+
+        setvar = self.interp.tk.setvar
+        default = StrRaises()
+        with self.assertRaisesRegex(RuntimeError, "setvar evil"):
+            setvar("name1", "name2", default)
+
     def test_user_command(self):
         result = None
         def testfunc(arg):

Misc/NEWS.d/next/Library/2026-11-05-12-29-04.gh-issue-161388.s7ECT8.rst

@@ -0,0 +1,2 @@
+Fixes a crash when passing objects with raising :meth:`~object.__str__`
+method to :meth:`~tkinter.Misc.setvar`.

Modules/_tkinter.c

@@ -1235,7 +1235,7 @@ AsObjNested(PyObject *value, int depth)
     {
         PyObject *v = PyObject_Str(value);
         if (!v)
-            return 0;
+            return NULL;
         result = AsObjNested(v, depth);
         Py_DECREF(v);
         return result;
@@ -2101,6 +2101,9 @@ SetVar(TkappObject *self, PyObject *args, int flags)
 
         /* XXX must hold tcl lock already??? */
         newval = AsObj(newValue);
+        if (newval == NULL) {
+            return NULL;
+        }
         if (self->trace) {
             if (flags & TCL_GLOBAL_ONLY) {
                 TRACE(self, ("((sssNO))", "uplevel", "#0", "set",