Python: F-String Slowdown Fix and Runtime Hardening

Core runtime work dominates, with a quadratic f-string tokenizer fix backported across three release branches, plus specialization and just-in-time compiler crash fixes. Smaller reliability updates touch logging, IDLE, and C extension tooling.

Duration: PT2M36S

Episode overview

This episode is a short developer briefing from Python.

It explains recent repository work in plain language.

  • Show: Python
  • Published: 2026-09-20T13:08:43Z
  • Audio duration: PT2M36S

Transcript excerpt

This excerpt keeps the crawler page concise. Listen to the episode or use the RSS feed for the full update.

Hello, it's Sunday, September 20th, 2026. Welcome to your Python daily briefing.

The lead insight: this cycle is about making the interpreter fast and predictable again, with backports carrying the same performance fix to every supported release.

First, f-string tokenization. PR 156756 tackles issue 155525, where files with many replacement fields triggered repeated copying of the remaining source — quadratic work. The fix keeps replacement expressions in the tokenizer buffer instead of recopying, preserves expression pointers when the buffer grows, and…

Second, specialization and the just-in-time compiler. PR 157834 fixes specialized C calls that accepted class, static and coexist flags, then failed their guards on every call. Only the method flag still changes the calling convention, so it stays checked. Separately, PR 157855 fixes a crash when multiple threads…

Third, everyday reliability. PR 154542 makes file logging handlers respect the raise-exceptions setting when opening the file fails, instead of raising unconditionally. PR 157706 keeps the IDLE shell prompt with the input in progress when background thread output arrives. For introspection, PR 157488 adds libffi…

What's…

Nearby episodes from Python

  1. Cleanup, Correctness, and a Trickle of Backports
  2. Hardening Memory Safety at the Core
  3. IDLE Gets a Major Reliability Sweep
  4. Hardening the Core Against Silent Corruption
  5. Parser Speed and Memory Safety Sweep
  6. XML Cleanup, Parser Speedups, and a Docs Split
  7. Weekly Recap - Memory Safety, Bytes Internals, and Doc Restructuring
  8. Hardening the Bytes Core and Trimming the Parser