gh-149046: fix: correctly handle str subclasses in StringIO

Lib/test/test_io/test_memoryio.py

@@ -967,6 +967,25 @@ def test_setstate(self):
         memio.close()
         self.assertRaises(ValueError, memio.__setstate__, ("closed", "", 0, None))
 
+    def test_write_str_subclass(self):
+        # Writing a str subclass should use the subclass's unicode data
+        # directly, not call __str__ on it (which may return a different
+        # value).  gh-149047
+        class MyStr(str):
+            def __str__(self):
+                return "WRONG"
+
+        s = MyStr("correct")
+        memio = self.ioclass()
+        memio.write(s)
+        self.assertEqual(memio.getvalue(), "correct")
+
+        # Also test the fast path where pos == string_size (STATE_ACCUMULATING)
+        memio2 = self.ioclass()
+        memio2.write(MyStr("hello "))
+        memio2.write(MyStr("world"))
+        self.assertEqual(memio2.getvalue(), "hello world")
+
 
 class CStringIOPickleTest(PyStringIOPickleTest):
     UnsupportedOperation = io.UnsupportedOperation

Misc/NEWS.d/next/Library/2026-04-27-11-12-00.gh-issue-149046.74shDd.rst

@@ -0,0 +1,2 @@
+:mod:`io`: Fix :class:`io.StringIO` serialization: no longer call ``str(obj)`` on :class:`str`
+subclasses. Patch by Thomas Kowalski.

Modules/_io/stringio.c

@@ -225,7 +225,7 @@ write_str(stringio *self, PyObject *obj)
 
     if (self->state == STATE_ACCUMULATING) {
         if (self->string_size == self->pos) {
-            if (PyUnicodeWriter_WriteStr(self->writer, decoded))
+            if (_PyUnicodeWriter_WriteStr((_PyUnicodeWriter*)self->writer, decoded))
                 goto fail;
             goto success;
         }