Mucha paintaing: the 4 seasons

In Python 3.13 alpha 1, I removed more than 300 private C API functions. Even if I announced my plan early in July, users didn't "embrace" my plan and didn't agree with the rationale. I reverted 50 functions in the alpha 2 release to calm down the situation and have more time to replace private functions with public functions.

Painting: The Seasons by Czech visual artist Alphonse Mucha (1900)

Remove private functions

On June 25th, I created issue gh-106084: "Remove private C API functions from abstract.h".

Over the years, we accumulated many private functions as part of the public C API in abstract.h header file. I propose to remove them: move them to the internal C API.

On July 1st, I created the meta issue gh-106320: "Remove private C API functions". The issue has 63 pull requests (a lot!), 53 comments and more than 300 events (created by commits and pull requests) which make the issue hard to navigate.

On July 3rd, Petr Viktorin shared his concerns:

Please be careful about assuming that the underscore means a function is private. AFAIK, that rule first appears for 3.10, and was only properly formalized in PEP 689, for Python 3.12.

For older functions, please consider if they should be added to the unstable API. IMO it's better to call them “underscored” than “private”.

See also: historical note in the devguide.

On July 4th, Petr posted on Discourse: (pssst) Let's treat all API in public headers as public.

Remove more private functions

On July 4th, I removed 181 private functions so far.

On July 4th, I identified that 34 projects on PyPI top 5,000 are affected by these removals.

On July 7th, I added PyObject_Vectorcall() to the pythoncapi-compat project.

On July 9th, I started the discussion: C API: How much private is the private _Py_IDENTIFIER() API?

On July 13th, I asked if the PyComplex API should be made private or not. Petr noticed that this API was documented.

On July 23th, I tried to build numpy, but I was blocked by Cython which was broken by my changes. I created the issue gh-107076: "C API: Cython 3.0 uses private functions removed in Python 3.13 (numpy 1.25.1 fails to build)".

On July 23th, I found that the private _PyTuple_Resize() function is documented. I proposed adding a new internal _PyTupleBuilder API to replace _PyTuple_Resize().

On July 23th, I proposed: C API: My plan to clarify private vs public functions in Python 3.13.

Private API has multiple issues: they are usually not documented, not tested, and so their behavior may change without any warning or anything. Also, they can be removed anytime without any notice.
  • Phase 1: Remove as many private API as possible
  • Phase 2 (Python 3.13 alpha 1): revert removals if needed to make sure that Cython, numpy and pip work.
  • Phase 3 (Python 3.13 beta 1): consider reverting more removals if needed.

On July 24th, I created the PR Remove private _PyCrossInterpreterData API. Eric Snow asked me to keep this private API since it's used by 3rd party C extensions.

On August 24th, I created issue gh-108444 to add PyLong_AsInt() public function, replacing the removed _PyLong_AsInt() function.

On September 4th, I looked at the _PyArg API. I started the discussion: Use the limited C API for some of our stdlib C extensions.

On September 4th, I declared:

I declare that the Python 3.13 season of “removing as many private C API as possible” ended! I stop here until Python 3.14.

Python 3.12 exports 385 private functions. After the cleanup, Python 3.13 only exported 86 private functions: I removed 299 functions. I closed the issue.

Python 3.13 alpha 1 negative feedback

On October 13th, Python 3.13 alpha 1 was released with my changes removing around 300 private C API functions.

On October 14th, Guido van Rossum asked:

Thanks for the list. Should we encourage various projects to test 3.13a1, which just came out? Is there a way we can encourage them more?

On October 30th, Stefan Behnel, Cython creator, posted the message: Python 3.13 alpha 1 contains breaking changes, what's the plan?. He also commented the issue. Extract:

I just came across this issue. Let me express my general disapproval regarding deliberate breakage, which this issue appears to be entirely about. As far as I can see, none of these removals was motivated. The mere idea of removing existing API "because we can" is entirely foreign to me.

On October 31th, Petr asked the Steering Council: Is it OK to remove _PyObject_Vectorcall? about the removal of old aliases with underscore, such as _PyObject_Vectorcall. I didn't know that these names were part of PEP 590 – Vectorcall: a fast calling protocol for CPython, nothing was written about that in the header files.

On November 2nd, Guido wrote (where WG stands for C API Working Group):

We can talk till we’re blue in the face but please no more action (i.e., no more moving/removing APIs) until the full WG has had a chance to discuss this and make a decision.

(Restoring removed APIs at users’ requests is fine.)

On November 3rd, Gregory P. Smith wrote:

I'd much prefer 'revert' for any API anyone is found using in 3.13.

We need to treat 3.13 as a more special than usual release and aim to minimize compatibility headaches for existing project code. That way more things that build and run on 3.12 build can run on 3.13 as is or with minimal work.

This will enable ecosystem code owners to focus on the bigger picture task of enabling existing code to be built and tested on an experimental pep703 free-threading build rather than having a pile of unrelated cleanup trivia blocking that.

On November 7th, my colleague Karolina Surma posted a report: Ongoing packages' rebuild with Python 3.13 in Fedora. She did a great bug triage work on counting build failures per C API issue by recompiling 4000+ Python packages in Fedora with Python 3.13.

On November 13th, Petr also identified that the private PyComplex API, such as _Py_c_sum() function, was documented. Moreover, the issue gh-112019 was created to ask to revert these APIs.

Revert in Python 3.13 alpha 2

On November 13th, I created issue gh-112026: "[C API] Revert of private functions removed in Python 3.13 causing most problems". I made 4 changes:

  • Add again <unistd.h> include in Python.h
  • Restore removed private C API
  • Restore removed _PyDict_GetItemStringWithError()
  • Add again _PyThreadState_UncheckedGet() function

I selected functions by looking at bug reports, Karolina's report, and by trying to build numpy and cffi. With my reverts, numpy built successfully, and cffi built successfully with a minor change that I reported upstream (cffi: Use PyErr_FormatUnraisable() on Python 3.13).

In total, I restored 50 private functions.

On November 22th, Python 3.13 alpha 2 was released with these restored functions. It seems like the situation is calmer now.

Reverting was part of my initial plan, it was clearly announced since the beginning. But I didn't expect that so many people would test Python 3.13 alpha 1 as soon as it was released (October)! Usually, we only start to get feedback around beta 1 (May). I had like 2 weeks to fix most issues instead of 7 months. It was really stressful for me.

I posted a message to apologize and to give the context of this work. Extract:

Following the announced plan 22, I reverted 50 private APIs 20 which were removed in Python 3.13 alpha 1. These APIs will be available again in the incoming Python 3.13 alpha 2 (scheduled next Tuesday).

I planned to make Cython, numpy and cffi compatible with Python 3.13 alpha 1. Well, I missed this release. With reverted changes, numpy 1.26.2 can be built successfully, and cffi 1.16.0 just requires a single change 13. So we should be good (or almost good) for Python 3.13 alpha 2.


I’m sorry if some people felt that this C API work was forced on them and their opinion was not taken in account. We heard you and we took your feedback in account. It took me time to adjust my plan according to early received feedback. I expected to have 6 months to work step by step. Well, I had 2 weeks instead 🙂

Add public functions

On October 30th, I created issue gh-111481: "[C API] Meta issue: add new public functions with doc+tests to replace removed private functions".

So far, I added 7 public functions to Python 3.13:

  • PyDict_Pop()
  • PyDict_PopString()
  • PyList_Clear()
  • PyList_Extend()
  • PyLong_AsInt()
  • Py_HashPointer()
  • Py_IsFinalizing()

More functions are coming soon, I have many open pull requests!

Adding new functions is slower than what I expected. The good part is that many people are reviewing the APIs, and that the new public APIs are way better than the old private ones: less error prone, can be more efficient, etc. At least, the conversion of private to public is moving steadily, functions are added one by one.