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

mohsinm-dev · 2025-11-10T01:52:24Z

Summary

Fix assertion failure in io.TextIOWrapper.tell() when reading files ending with standalone \r
Add comprehensive test case to prevent regression
Add NEWS entry documenting the fix

Root Cause

The tell() optimization algorithm assumed buffered data would always be available when calculating position, but files ending with standalone \r create empty snapshots, causing assert(skip_back <= PyBytes_GET_SIZE(next_input)) to fail.

Fix

Added check to skip optimization when next_input is empty, preventing the assertion failure while maintaining correct position calculation.

Test Plan

Added test case test_tell_after_readline_with_cr in test_textio.py
Verified fix handles standalone carriage returns correctly
Confirmed all existing io tests still pass
Built CPython with debug assertions enabled

Issue: Assertion failure in Modules/_io/textio.c _io_TextIOWrapper_tell_impl: Assertion 'skip_back <= PyBytes_GET_SIZE(next_input)' failed #141314

…dalone carriage return When TextIOWrapper.tell() is called after reading a line that ends with a standalone carriage return (\r), the tell optimization algorithm incorrectly assumes there is buffered data to search through. This causes an assertion failure when skip_back=1 exceeds the empty buffer size. The fix detects when next_input is empty and skips the optimization phase, falling back to the byte-by-byte decoding method which always works correctly. This properly handles the architectural constraint that buffer optimization cannot function without buffered data.

…dalone carriage return Add test case and fix assertion failure in TextIOWrapper.tell() when reading files that end with a standalone carriage return (\r). The optimization algorithm incorrectly assumed buffered data would always be available, causing an assertion failure when next_input is empty.

- Remove trailing whitespace - Add missing newline at end of NEWS file

Modules/_io/textio.c

Lib/test/test_io/test_textio.py

serhiy-storchaka

The cause of this bug is a simple typo.

    assert(skip_bytes <= PyBytes_GET_SIZE(next_input));

serhiy-storchaka · 2025-11-10T13:40:46Z

Lib/test/test_io/test_textio.py

+            remaining = f.read()
+            self.assertEqual(remaining, "")
+
+    def test_tell_after_readline_with_multiple_cr(self):


Is this test necessary?

serhiy-storchaka · 2025-11-10T13:42:17Z

Lib/test/test_io/test_textio.py

+    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=1\r'


It can be simply b'line1\r'

bedevere-app · 2025-11-10T13:43:21Z

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

sergey-miryanov

Please revert unnecessary changes in test_textio.py

- Fix assertion to check skip_bytes instead of skip_back - Use simpler test data b'line1\r' instead of b'line1=1\r' - Remove unnecessary multiple CR test case - Clean up workaround code The assertion was checking wrong variable (skip_back vs skip_bytes). skip_back is search step size, skip_bytes is buffer offset needing validation.

serhiy-storchaka

LGTM, 👍

sergey-miryanov

Thanks!

miss-islington-app · 2025-11-11T21:50:21Z

Thanks @mohsinm-dev for the PR, and @serhiy-storchaka for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

miss-islington-app · 2025-11-11T21:50:22Z

Thanks @mohsinm-dev for the PR, and @serhiy-storchaka for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

miss-islington-app · 2025-11-11T21:50:24Z

Sorry, @mohsinm-dev and @serhiy-storchaka, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker af80fac42548719ede7241bfbab3c2c0775b4760 3.13

miss-islington-app · 2025-11-11T21:50:28Z

Sorry, @mohsinm-dev and @serhiy-storchaka, I could not cleanly backport this to 3.14 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker af80fac42548719ede7241bfbab3c2c0775b4760 3.14

cmaloney · 2025-11-12T07:18:02Z

The cherry picks conflicts are probably because of the io test movement, happy to work on them if you'd like

mohsinm-dev · 2025-11-12T09:41:45Z

The cherry picks conflicts are probably because of the io test movement, happy to work on them if you'd like

If I am not wrong Lib/test/test_io/test_textio.py exists in main branch (where our commit adds tests). But it was deleted in 3.13 branch and 3.14 branch because the tests are still in test_general.py

bedevere-app · 2025-11-12T09:44:21Z

GH-141452 is a backport of this pull request to the 3.13 branch.

bedevere-app · 2025-11-12T09:44:23Z

GH-141453 is a backport of this pull request to the 3.14 branch.

…th standalone carriage return (pythonGH-141331) The assertion was checking wrong variable (skip_back vs skip_bytes). (cherry picked from commit af80fac) Co-authored-by: Mohsin Mehmood <55545648+mohsinm-dev@users.noreply.github.com>

bedevere-app · 2025-11-12T10:13:16Z

GH-141452 is a backport of this pull request to the 3.13 branch.

bedevere-app · 2025-11-12T10:14:18Z

GH-141453 is a backport of this pull request to the 3.14 branch.

efimov-mikhail · 2025-11-12T10:30:51Z

IMO, these backports are correct and we can merge them.

…ndalone carriage return (GH-141331) (GH-141453) The assertion was checking wrong variable (skip_back vs skip_bytes). (cherry picked from commit af80fac)

…ndalone carriage return (GH-141331) (GH-141452) The assertion was checking wrong variable (skip_back vs skip_bytes). (cherry picked from commit af80fac)

mohsinm-dev added 2 commits November 10, 2025 06:31

bedevere-app bot added the awaiting review label Nov 10, 2025

bedevere-app bot mentioned this pull request Nov 10, 2025

Assertion failure in Modules/_io/textio.c _io_TextIOWrapper_tell_impl: Assertion 'skip_back <= PyBytes_GET_SIZE(next_input)' failed #141314

Closed

Fix linting issues

cdb9283

- Remove trailing whitespace - Add missing newline at end of NEWS file

sergey-miryanov reviewed Nov 10, 2025

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mohsinm-dev added 2 commits November 10, 2025 14:30

Refactor code structure and add test for multiple CR cases

b1e1885

Fix trailing whitespace in test file

1d9f77f

serhiy-storchaka self-requested a review November 10, 2025 13:14

serhiy-storchaka requested changes Nov 10, 2025

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mohsinm-dev force-pushed the fix-textio-tell-assertion-gh-141314 branch from b298058 to e84d686 Compare November 10, 2025 17:53

serhiy-storchaka approved these changes Nov 11, 2025

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bedevere-app bot added awaiting merge and removed awaiting changes labels Nov 11, 2025

efimov-mikhail approved these changes Nov 11, 2025

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sergey-miryanov approved these changes Nov 11, 2025

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serhiy-storchaka merged commit af80fac into python:main Nov 11, 2025
49 of 50 checks passed

bedevere-app bot removed the awaiting merge label Nov 11, 2025

serhiy-storchaka added needs backport to 3.13 bugs and security fixes needs backport to 3.14 bugs and security fixes labels Nov 11, 2025

miss-islington-app bot assigned serhiy-storchaka Nov 11, 2025

This was referenced Nov 12, 2025

[3.13] gh-141314: Fix TextIOWrapper.tell() assertion failure with standalone carriage return (GH-141331) #141452

Merged

[3.14] gh-141314: Fix TextIOWrapper.tell() assertion failure with standalone carriage return (GH-141331) #141453

Merged

bedevere-app bot removed the needs backport to 3.13 bugs and security fixes label Nov 12, 2025

bedevere-app bot removed the needs backport to 3.14 bugs and security fixes label Nov 12, 2025

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

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

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