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

[mohsinm-dev]

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

Summary by CodeRabbit

• Bug Fixes

  • Fixed an assertion when reporting file position while reading files that include standalone carriage return (CR) line endings, improving stability of position reporting and seek/read interactions.

• Tests

  • Added a test that verifies reading a line with a standalone CR and subsequent tell/seek/read behavior.
  • Minor stylistic and formatting updates to tests.

[coderabbitai]

Walkthrough

Adds a new test that reproduces reading a line ending with a standalone CR and exercising TextIOWrapper.tell/seek/read, updates the C tell implementation assertion to validate skip_bytes, and adds a NEWS entry documenting the bugfix for an assertion failure with \r line endings.

Changes

Cohort / File(s)	Summary
Tests Lib/test/test_io/test_textio.py	Adds test_tell_after_readline_with_cr: writes b'line1\r', reopens in text mode, reads a line asserting "line1\n", calls tell(), seeks to that position, reads remaining content and asserts it's empty.
C implementation (I/O) Modules/_io/textio.c	In _io_TextIOWrapper_tell_impl(), changed the internal validation to assert on skip_bytes (the number of bytes used to start decoding) instead of skip_back. No API changes.
Changelog Misc/NEWS.d/next/...gh-issue-141314.baaa28.rst	Added NEWS entry describing a bugfix: fixed an assertion failure in io.TextIOWrapper.tell() when reading files with standalone \r line endings.

Sequence Diagram(s)

sequenceDiagram
    autonumber
    participant Test as Test file
    participant PyAPI as TextIOWrapper (Python)
    participant CImpl as _io_TextIOWrapper_tell_impl (C)

    Test->>PyAPI: open file, readline()
    PyAPI->>CImpl: compute internal buffer/positions
    CImpl->>CImpl: compute/validate skip_bytes (was skip_back)
    Note over CImpl: assertion now checks `skip_bytes`
    CImpl-->>PyAPI: return position
    PyAPI-->>Test: tell()/seek()/read() results used by test

Loading

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

• Inspect Modules/_io/textio.c change for correctness around buffer/position math and boundary cases (standalone \r).
• Verify the new test reliably reproduces the scenario and is robust across platforms/encodings.
• Confirm NEWS entry wording and placement.

Poem

I nibble bytes with whiskered care,
A lone CR found me in the lair,
I check the skips and count each hop,
No more assertions — hop, hop, hop! 🐇

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 25.00% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title directly reflects the main change: fixing an assertion failure in TextIOWrapper.tell() with standalone carriage returns, which is the core issue addressed across all modified files.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix-textio-tell-assertion-gh-141314

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📜 Recent review details

Configuration used: CodeRabbit UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between b298058 and e84d686.

📒 Files selected for processing (2)

• Lib/test/test_io/test_textio.py (1 hunks)
• Modules/_io/textio.c (1 hunks)

🚧 Files skipped from review as they are similar to previous changes (1)

• Modules/_io/textio.c

🔇 Additional comments (1)

Lib/test/test_io/test_textio.py (1)

689-706: Excellent regression test!

This test effectively reproduces the pythongh-141314 assertion failure scenario and verifies the fix. The test structure is clear and follows the conventions in this file.

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