CPython Internals Explained • Hacker News Board

Re: CPython Internals Explained

#51

Post by lozenge » Sun, Feb 01, 2026, 11:08 AM UTC

EdwardDiego wrote: post #39

squeedles wrote: post #21

Had to write a fairly substantial native extension to Python a couple years ago and one of the things I enjoyed was that the details were not easily "Googleable" because implementation results were swamped by language level results. It took me back to the old days of source diving and accumulated knowledge that you carried around in your head. https://www.dave.org/posts/20220806_python/

Great write up, thank you for sharing it! Quick question though, in your first code example (dynamic enum with a metaclass) what is "m" in this line towards the start? Py_DECREF(m) Is it the metaclass?

That's a standard error clause. In the case PyImport_ImportModule threw a Python exception, you need to Py_DECREF any C local variables which are new references(not borrowed references) and return -1.

From the later call PyModule_AddObject, it's clear this code has come from the PyInit_ module initialisation function. This code is running on import of the C extension to initialise the "FruitEnum" module attribute. https://docs.python.org/3/c-api/extension-modules.html#c.PyI...

Re: CPython Internals Explained

#52

Post by squeedles » Sun, Feb 01, 2026, 1:04 PM UTC

lozenge wrote: post #51

EdwardDiego wrote: post #39

Earlier quoted context omitted.

Great write up, thank you for sharing it! Quick question though, in your first code example (dynamic enum with a metaclass) what is "m" in this line towards the start? Py_DECREF(m) Is it the metaclass?

That's a standard error clause. In the case PyImport_ImportModule threw a Python exception, you need to Py_DECREF any C local variables which are new references(not borrowed references) and return -1. From the later call PyModule_AddObject, it's clear this code has come from the PyInit_ module initialisation function. This code is running on import of the C extension to initialise the "FruitEnum" module attribute. ht…

Exactly so. I didn't notice that missing def when I put together the blog post, but you are right to call it out. In this case that decref was copypasta from some other code -- I don't decref on the other error returns. I combined code that was in several places and omitted the decref for mod_enum too!

The module init function is where you would normally create the module object (PyModule_Create) and decref it if an error occurs. The blog example is utility code that you would call within the module init function to add an enum.

Someone should really create a blog post compiler to catch these sorts of things :-)

Re: CPython Internals Explained

#53

Post by westurner » Sun, Feb 01, 2026, 3:02 PM UTC

shakna wrote: post #46

westurner wrote: post #44

Earlier quoted context omitted.

That's why three different procedures for docs? Python docs procedures: (0) Devguide, (1) PEPs w/ front matter in RST, (2) RST in /Doc with Sphinx, (3) MD and TXT in /InternalDocs without a toctree The .. warning: or even admonition directives could be used for indicating that docs under /internals are not public API and can change with or without a PEP; though that should also or at least be indicated in the source…

How many, many times has a project said, "Don't use, internal only", only for it to become an industry-wide common "trick"? Saying "here be dragons" is not enough to discourage people whose job it is to be creative.

That's the bad kind of lazy.

It is advantageous to format, interlink, and create a table of contents for documentation. I doubt anyone would advocate for removing the Makefile and conf.py from /Doc.

That a document describes something internal does not mean that it should be excluded from the docs.

Is there a consistent standard for whether something is internal or external to the CPython project? Aren't there already internal things documented in /Doc? Should they be removed from the docs build then? Or moved to an internal folder with a DRY additional toctree?

Re: CPython Internals Explained

#54

Post by tonymet » Sun, Feb 01, 2026, 4:58 PM UTC

palata wrote: post #34

tonymet wrote: post #32

Earlier quoted context omitted.

I don’t think it’s good form to downvote people you disagree with.

I did not downvote, but I'm guessing that it is perceived as disrespectful to call them failures to the point where they don't even qualify as "alternatives".

It’s no slight to jython. They fill an important Niche. But jython will never ever be confused with python.