gh-161518: Reject a non-bytes encoder result unless it is the ASCII fast path

TextIOWrapper.write() accepted any encoder result that was the same
object as its input. That is only safe for ASCII text, so a custom
encoder returning a non-ASCII str wrote corrupt data.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Author: harjothkhara

Lib/test/test_io/test_textio.py

@@ -1097,6 +1097,31 @@ def test_illegal_encoder(self):
             t = self.TextIOWrapper(self.BytesIO(b'foo'), encoding="rot13")
         self.assertRaises(TypeError, t.write, 'bar')
 
+    def test_encoder_returns_str(self):
+        # gh-161518: an encoder returning its non-ASCII input str must be
+        # rejected, not have its internal storage written as bytes.
+        class Encoder(codecs.IncrementalEncoder):
+            def encode(self, input, final=False):
+                return input
+
+        info = codecs.CodecInfo(
+            None, None, incrementalencoder=Encoder,
+            incrementaldecoder=codecs.getincrementaldecoder('utf-8'),
+            name='test_str_encoder')
+        search = lambda name: info if name == 'test_str_encoder' else None
+        codecs.register(search)
+        self.addCleanup(codecs.unregister, search)
+
+        t = self.TextIOWrapper(self.BytesIO(), encoding='test_str_encoder')
+        self.assertRaises(TypeError, t.write, 'caf\xe9')
+
+        # The ASCII fast path still works.
+        raw = self.BytesIO()
+        t = self.TextIOWrapper(raw, encoding='utf-8')
+        t.write('cafe')
+        t.flush()
+        self.assertEqual(raw.getvalue(), b'cafe')
+
     def test_illegal_decoder(self):
         # Issue #17106
         # Bypass the early encoding check added in issue 20404

Misc/NEWS.d/next/Library/2026-11-06-21-50-00.gh-issue-161518.Tx9Kq2.rst

@@ -0,0 +1,2 @@
+:class:`io.TextIOWrapper` now raises :exc:`TypeError`, instead of writing
+corrupt output, when an encoder returns its non-ASCII input string.

Modules/_io/textio.c

@@ -1831,7 +1831,7 @@ _io_TextIOWrapper_write_impl(textio *self, PyObject *text)
     Py_DECREF(text);
     if (b == NULL)
         return NULL;
-    if (b != text && !PyBytes_Check(b)) {
+    if (!PyBytes_Check(b) && !(b == text && PyUnicode_IS_ASCII(text))) {
         PyErr_Format(PyExc_TypeError,
                      "encoder should return a bytes object, not '%.200s'",
                      Py_TYPE(b)->tp_name);