gh-161055: Fix TextIOWrapper.tell() after next() and flush()

Lib/_pyio.py

@@ -2271,7 +2271,10 @@ def writable(self):
 
     def flush(self):
         self.buffer.flush()
-        self._telling = self._seekable
+        # The snapshot is stale after next(), so only re-enable tell() when
+        # it needs neither the snapshot nor pending decoded text.
+        if self._snapshot is None and not self._decoded_chars:
+            self._telling = self._seekable
 
     def close(self):
         if self.buffer is not None and not self.closed:
@@ -2568,6 +2571,7 @@ def _reset_encoder(position):
             position = self.buffer.seek(0, whence)
             self._set_decoded_chars('')
             self._snapshot = None
+            self._telling = self._seekable
             if self._decoder:
                 self._decoder.reset()
             _reset_encoder(position)
@@ -2587,6 +2591,7 @@ def _reset_encoder(position):
         self.buffer.seek(start_pos)
         self._set_decoded_chars('')
         self._snapshot = None
+        self._telling = self._seekable
 
         # Restore the decoder to its state from the safe start point.
         if cookie == 0 and self._decoder:

Lib/test/test_io/test_textio.py

@@ -597,6 +597,29 @@ def test_telling(self):
         self.assertEqual(f.tell(), p2)
         f.close()
 
+    def test_tell_after_next_and_flush(self):
+        # next() stops updating the decoder snapshot, and flush() must not
+        # re-enable tell() with that stale snapshot.
+        data = b"a\n" + b"b" * 10000 + b"\nc\n"
+        for prepare in ["none", "readline", "seek"]:
+            with self.subTest(prepare=prepare):
+                f = self.TextIOWrapper(self.BytesIO(data), encoding="ascii")
+                if prepare != "none":
+                    self.assertEqual(f.readline(), "a\n")
+                if prepare == "seek":
+                    f.seek(f.tell())
+                next(f)
+                f.flush()
+                self.assertRaises(OSError, f.tell)
+                self.assertEqual(list(f)[-1], "c\n")
+                self.assertEqual(f.tell(), len(data))
+                f.seek(0)
+                next(f)
+                self.assertRaises(OSError, f.tell)
+                f.seek(2)
+                self.assertEqual(f.tell(), 2)
+                self.assertEqual(f.read(3), "bbb")
+
     def test_seeking(self):
         chunk_size = _default_chunk_size()
         prefix_size = chunk_size - 2

Misc/NEWS.d/next/Library/2026-11-01-02-30-51.gh-issue-161055.qPmPSU.rst

@@ -0,0 +1,3 @@
+Fix :meth:`io.TextIOWrapper.tell` returning a wrong position after
+:func:`next` and :meth:`~io.IOBase.flush`. It now raises :exc:`OSError`, as it
+does without the :meth:`!flush` call.

Modules/_io/textio.c

@@ -2712,6 +2712,7 @@ _io_TextIOWrapper_seek_impl(textio *self, PyObject *cookieObj, int whence)
 
         textiowrapper_set_decoded_chars(self, NULL);
         Py_CLEAR(self->snapshot);
+        self->telling = self->seekable;
         if (self->decoder) {
             res = PyObject_CallMethodNoArgs(self->decoder, &_Py_ID(reset));
             if (res == NULL)
@@ -2780,6 +2781,7 @@ _io_TextIOWrapper_seek_impl(textio *self, PyObject *cookieObj, int whence)
 
     textiowrapper_set_decoded_chars(self, NULL);
     Py_CLEAR(self->snapshot);
+    self->telling = self->seekable;
 
     /* Restore the decoder to its state from the safe start point. */
     if (self->decoder) {
@@ -3256,7 +3258,14 @@ _io_TextIOWrapper_flush_impl(textio *self)
 {
     CHECK_ATTACHED(self);
     CHECK_CLOSED(self);
-    self->telling = self->seekable;
+    /* The snapshot is stale after next(), so only re-enable tell() when
+       it needs neither the snapshot nor pending decoded text. */
+    if (self->snapshot == NULL &&
+        (self->decoded_chars == NULL ||
+         PyUnicode_GET_LENGTH(self->decoded_chars) == 0))
+    {
+        self->telling = self->seekable;
+    }
     if (_textiowrapper_writeflush(self) < 0)
         return NULL;
     return buffer_callmethod_noargs(self, &_Py_ID(flush));