gh-160185: Fix CJK StreamWriter.reset() not returning to ASCII

Lib/codecs.py

@@ -402,9 +402,10 @@ def reset(self):
         pass
 
     def seek(self, offset, whence=0):
-        self.stream.seek(offset, whence)
+        # reset() may write to the stream: call it at the old position.
         if whence == 0 and offset == 0:
             self.reset()
+        self.stream.seek(offset, whence)
 
     def __getattr__(self, name,
                     getattr=getattr):
@@ -741,10 +742,11 @@ def reset(self):
         self.writer.reset()
 
     def seek(self, offset, whence=0):
-        self.stream.seek(offset, whence)
-        self.reader.reset()
+        # writer.reset() may write to the stream: call it at the old position.
         if whence == 0 and offset == 0:
             self.writer.reset()
+        self.stream.seek(offset, whence)
+        self.reader.reset()
 
     def __getattr__(self, name,
                     getattr=getattr):
@@ -864,8 +866,8 @@ def reset(self):
     def seek(self, offset, whence=0):
         # Seeks must be propagated to both the readers and writers
         # as they might need to reset their internal buffers.
-        self.reader.seek(offset, whence)
         self.writer.seek(offset, whence)
+        self.reader.seek(offset, whence)
 
     def __getattr__(self, name,
                     getattr=getattr):

Lib/test/test_multibytecodec.py

@@ -438,6 +438,42 @@ def test_incrementalencoder_final(self):
         self.assertEqual(output, self.expected_reset)
         self.assertEqual(encoder.encode('', final=True), b'')
 
+    def test_streamwriter_reset(self):
+        stream = io.BytesIO()
+        writer = codecs.getwriter(self.encoding)(stream)
+        writer.reset()
+        self.assertEqual(stream.getvalue(), b'')
+        writer.write(self.text)
+        self.assertEqual(stream.getvalue(), self.expected)
+        writer.reset()
+        self.assertEqual(stream.getvalue(), self.expected_reset)
+        writer.reset()
+        self.assertEqual(stream.getvalue(), self.expected_reset)
+        writer.write(self.text)
+        writer.reset()
+        self.assertEqual(stream.getvalue(), self.expected_reset * 2)
+
+    def test_streamwriter_seek0(self):
+        # The reset sequence is written before the stream is rewound.
+        info = codecs.lookup(self.encoding)
+        for wrap in (
+            info.streamwriter,
+            lambda stream: codecs.StreamReaderWriter(
+                stream, info.streamreader, info.streamwriter),
+            lambda stream: codecs.EncodedFile(stream, 'utf-8', self.encoding),
+        ):
+            stream = io.BytesIO()
+            writer = wrap(stream)
+            text = self.text
+            if isinstance(writer, codecs.StreamRecoder):
+                text = text.encode()
+            writer.write(text)
+            writer.seek(0)
+            self.assertEqual(stream.tell(), 0)
+            self.assertEqual(stream.getvalue(), self.expected_reset)
+            writer.write(text)
+            self.assertEqual(stream.getvalue(), self.expected_reset)
+
 class TestHZStateful(TestStateful):
     text = '\u804a\u804a'
     encoding = 'hz'

Misc/NEWS.d/next/Library/2026-10-22-00-29-55.gh-issue-160185.BPz40E.rst

@@ -0,0 +1,5 @@
+Fix :meth:`codecs.StreamWriter.reset` of the stateful CJK codecs (``hz`` and
+the ``iso2022_*`` family) not writing the sequence that returns the output to
+ASCII.  ``seek(0)`` of :class:`codecs.StreamWriter`,
+:class:`codecs.StreamReaderWriter` and :class:`codecs.StreamRecoder` now
+resets the writer before moving the stream position.

Modules/cjkcodecs/multibytecodec.c

@@ -1857,13 +1857,17 @@ _multibytecodec_MultibyteStreamWriter_reset_impl(MultibyteStreamWriterObject *se
                                                  PyTypeObject *cls)
 /*[clinic end generated code: output=32ef224c2a38aa3d input=28af6a9cd38d1979]*/
 {
-    PyObject *pwrt;
+    PyObject *pwrt, *pending;
 
-    if (!self->pending)
-        Py_RETURN_NONE;
+    /* even without pending characters, a stateful codec may have to
+     * write a sequence that returns the output to its initial state */
+    pending = self->pending;
+    if (pending == NULL) {
+        pending = Py_GetConstantBorrowed(Py_CONSTANT_EMPTY_STR);
+    }
 
     pwrt = multibytecodec_encode(self->codec, &self->state,
-                    self->pending, NULL, self->errors,
+                    pending, NULL, self->errors,
                     MBENC_FLUSH | MBENC_RESET);
     /* some pending buffer can be truncated when UnicodeEncodeError is
      * raised on 'strict' mode. but, 'reset' method is designed to
@@ -1886,6 +1890,7 @@ _multibytecodec_MultibyteStreamWriter_reset_impl(MultibyteStreamWriterObject *se
             Py_DECREF(pwrt);
             return NULL;
         }
+        Py_DECREF(wr);
     }
     Py_DECREF(pwrt);