gh-141314: Fix TextIOWrapper.tell() assertion failure with standalone carriage return (GH-141331)

The assertion was checking wrong variable (skip_back vs skip_bytes).

Author: mohsinm-dev

Lib/test/test_io/test_textio.py

@@ -686,6 +686,25 @@ def test_multibyte_seek_and_tell(self):
         self.assertEqual(f.tell(), p1)
         f.close()
 
+    def test_tell_after_readline_with_cr(self):
+        # Test for gh-141314: TextIOWrapper.tell() assertion failure
+        # when dealing with standalone carriage returns
+        data = b'line1\r'
+        with self.open(os_helper.TESTFN, "wb") as f:
+            f.write(data)
+
+        with self.open(os_helper.TESTFN, "r") as f:
+            # Read line that ends with \r
+            line = f.readline()
+            self.assertEqual(line, "line1\n")
+            # This should not cause an assertion failure
+            pos = f.tell()
+            # Verify we can seek back to this position
+            f.seek(pos)
+            remaining = f.read()
+            self.assertEqual(remaining, "")
+
+
     def test_seek_with_encoder_state(self):
         f = self.open(os_helper.TESTFN, "w", encoding="euc_jis_2004")
         f.write("\u00e6\u0300")

Misc/NEWS.d/next/Library/2025-11-10-01-47-18.gh-issue-141314.baaa28.rst

@@ -0,0 +1 @@
+Fix assertion failure in :meth:`io.TextIOWrapper.tell` when reading files with standalone carriage return (``\r``) line endings.

Modules/_io/textio.c

@@ -2845,7 +2845,7 @@ _io_TextIOWrapper_tell_impl(textio *self)
        current pos */
     skip_bytes = (Py_ssize_t) (self->b2cratio * chars_to_skip);
     skip_back = 1;
-    assert(skip_back <= PyBytes_GET_SIZE(next_input));
+    assert(skip_bytes <= PyBytes_GET_SIZE(next_input));
     input = PyBytes_AS_STRING(next_input);
     while (skip_bytes > 0) {
         /* Decode up to temptative start point */