gh-160811: fix TextIOWrapper.tell() after seek() past a carriage return

Lib/_pyio.py

@@ -2602,6 +2602,10 @@ def _reset_encoder(position):
             self._set_decoded_chars(
                 self._decoder.decode(input_chunk, need_eof))
             self._snapshot = (dec_flags, input_chunk)
+            if self._decoded_chars:
+                self._b2cratio = len(input_chunk) / len(self._decoded_chars)
+            else:
+                self._b2cratio = 0.0
 
             # Skip chars_to_skip of the decoded characters.
             if len(self._decoded_chars) < chars_to_skip:

Lib/test/test_io/test_textio.py

@@ -705,6 +705,22 @@ def test_tell_after_readline_with_cr(self):
             remaining = f.read()
             self.assertEqual(remaining, "")
 
+    def test_tell_after_seek_past_cr(self):
+        # seek() to a position after a carriage return must not leave a
+        # stale bytes-per-char ratio behind for tell().
+        for data in (b'one\rtwo\rthree\r', 'ab\r\u20ac\u20ac\u20ac'.encode()):
+            expected = data.decode().replace('\r', '\n')
+            for i in range(len(expected)):
+                with self.subTest(data=data, i=i):
+                    f = self.TextIOWrapper(self.BytesIO(data),
+                                           encoding='utf-8')
+                    f.read(i)
+                    pos = f.tell()
+                    f.read(1)
+                    f.seek(pos)
+                    self.assertEqual(f.read(1), expected[i])
+                    f.seek(f.tell())
+                    self.assertEqual(f.read(), expected[i + 1:])
 
     def test_seek_with_encoder_state(self):
         f = self.open(os_helper.TESTFN, "w", encoding="euc_jis_2004")

Misc/NEWS.d/next/Library/2026-10-28-08-12-33.gh-issue-160811.wQ3xT7.rst

@@ -0,0 +1,4 @@
+Fix :meth:`io.TextIOWrapper.tell` after :meth:`~io.TextIOWrapper.seek` to a
+position that follows a carriage return.  The bytes-per-char ratio was not
+updated when the input chunk was replayed, so ``tell()`` could read past an
+internal buffer and return a wrong position.

Modules/_io/textio.c

@@ -2812,6 +2812,7 @@ _io_TextIOWrapper_seek_impl(textio *self, PyObject *cookieObj, int whence)
             goto fail;
         }
 
+        Py_ssize_t nbytes = PyBytes_GET_SIZE(input_chunk);
         snapshot = Py_BuildValue("iN", cookie.dec_flags, input_chunk);
         if (snapshot == NULL) {
             goto fail;
@@ -2824,7 +2825,14 @@ _io_TextIOWrapper_seek_impl(textio *self, PyObject *cookieObj, int whence)
         if (check_decoded(decoded) < 0)
             goto fail;
 
+        Py_ssize_t nchars = PyUnicode_GET_LENGTH(decoded);
         textiowrapper_set_decoded_chars(self, decoded);
+        if (nchars > 0) {
+            self->b2cratio = (double) nbytes / nchars;
+        }
+        else {
+            self->b2cratio = 0.0;
+        }
 
         /* Skip chars_to_skip of the decoded characters. */
         if (PyUnicode_GetLength(self->decoded_chars) < cookie.chars_to_skip) {