Victor Stinner blog 3

In November 2025 at Pycon France (at Lyon), I watched the talk "Build a frozendict type (immutable dictionary)" by Antoine Rozo (video in French). It was a funny talk building a frozendict type in pure Python exploring different implementations such as a frozen dataclass with slots, frozenset, and super. See …

I'm writing an article series on Free Threading internals to learn more about Free Threading, explain how it works, and explain how it solved the "remove the GIL" issue where previous attempts failed.

• 1. Reference counting
• 2. Deferred reference counting
• 3. PyMutex (this article)

PEP 703 replaces the unique Global Interpreter Lock (GIL …

I'm writing an article series on Free Threading internals to learn more about Free Threading, explain how it works, and explain how it solved the "remove the GIL" issue where previous attempts failed.

• 1. Reference counting
• 2. Deferred reference counting (this article)
• 3. PyMutex

In the previous article, we have seen how the …

I'm writing an article series on Free Threading internals to learn more about Free Threading, explain how it works, and explain how it solved the "remove the GIL" issue where previous attempts failed.

• 1. Reference counting (this article)
• 2. Deferred reference counting
• 3. PyMutex

This first article explores the problem of tracking objects …

In the Python C API, I dislike APIs modifying immutable objects such as _PyBytes_Resize(). I designed a whole new PyBytesWriter API to replace this _PyBytes_Resize() function. As usual in Python, it took multiple iterations and one year to design the API and to reach an agreement.

Picture: The Secret World …

Design an API can take time. This article describes the design of the C API to import and export Python integers. It takes place between August 2023 and December 2024. In total, the discussions got more than 448 messages!

The API is a thin abstraction on top of CPython implementation …

Here is a report on my 18 commits merged into Python in February 2025:

• Reorganize C API tests
• Use PyErr_FormatUnraisable()
• Reorganize includes
• C API: Remove PySequence_Fast()
• C API: Fix function signatures
• C API: Deprecate private _PyUnicodeWriter
• Documentation
• Misc changes

Painting: Le Déjeuner des Canotiers (1881) by Auguste Renoir.

In May, I designed a new C API to build a Python str object: the PyUnicodeWriter API. Many people were involved in the design and the discussion was quite long. The C API Working Group helped to design a better and more convenient API. It took me basically a whole …

In Python 3.13, I made multiple enhancements to make the limited C API more usable:

• Add 14 functions to the limited C API.
• Make the special debug build Py_TRACE_REFS compatible with the limited C API.
• Enhance Argument Clinic to generate C code using the limited C API.
• Add an …