Antonio Cuni's blog

Inside SPy, part 1: Motivations and Goals

Wed, 29 Oct 2025 17:13:42 +0000

Inside SPy🥸, part 1: Motivations and Goals

This is the first of a series of posts in which I will try to give a deep explanation ofSPy, including motivations, goals, rules of thelanguage, differences with Python and implementation details.

This post focuses primarily on the problem space: why Python is fundamentally hardto optimize, what trade-offs existing solutions require, and where current approachesfall short. Subsequent posts in this series will explore the solutions in depth. Fornow, let's start with the essential question: what is SPy?

!!! Success "" Before diving in, I want to express my gratitude to my employer, Anaconda, for giving me the opportunity to dedicate 100% of my time to this open-source project.

Slides for my EuroPython 2025 talks

Wed, 24 Sep 2025 22:48:34 +0000

EuroPython 2025 slides

Here are the slides for the three speeches which I gave atEuroPython 2025:

Tracing JITs in the real world @ CPython Core Dev Sprint

Wed, 24 Sep 2025 22:48:34 +0000

Tracing JITs in the real world @ CPython Core Dev Sprint

Last week I got to take part in the CPython Core Developer Sprint inCambridge, hosted by ARM and brilliantlyorganized by Diego Russo-- about ~50 core devs and guests were there, and I was excited to join as oneof the guests.

I had three main areas of focus:

C API: this was a follow up of what we discussed at the C API summit at EuroPython. The current C API is problematic, so we are exploring ideas for the development of PyNI (Python Native Interface), whose design will likely be heavily inspired by HPy. It's important to underline that this is just the beginning and the entire process will require multiple PEPs.
fancycompleter This is a small PR which I started months ago, to enable colorful tab completions within the Python REPL. I wrote the original version of fancycompleter 15 years ago, but colorful completions work only in combination with PyREPL. Now PyREPL is part of the standard library and enabled by default, so we can finally upstream it. I hope to see it merged soon.
"JIT stuff": I spent a considerable amount of time talking to the people who are working on the CPython JIT (in particular Mark, Brandt, Savannah, Ken Jin and Diego). Knowledge transfer worked in both ways: I learned a lot about the internal details of CPython's JIT, and conversely I shared with them some of the experience, pain points and gut feelings which I got by working many years on PyPy.

In particular, on the first day I presented a talk titled Tracing JIT and real world Python (slides and source code).

What follows is an annotated version of the slides.

SPy @ PyCon IT 2025

Wed, 24 Sep 2025 20:55:51 +0000

SPy @ PyCon IT 2025

Yesterday I talked about SPy at PyCon Italy.

Thanks to Hugo van Kemenade for the picture.

Slides are available here.

The edited video is not available yet, but in the meantime it's possible towatch unedited version available inside thelive streamat minute 05:44:05.

Inside CPython's attribute lookup

Mon, 25 Aug 2025 14:43:48 +0000

Inside CPython's attribute lookup

Python's attribute lookup logic seems pretty simple at a first glance: "firstlook in the instance __dict__, then look in its type".

However, the actual logic is much more complex because it needs to take intoaccount the descriptor protocol, the difference between lookups on instancesvs types, and what happens in presence of metaclasses.

Recently I implemented preliminary support for the descriptor protocol in SPy,which led me to investigate the CPython source code to get abetter grasp on the details. This is a write up on what I found, with links tothe actual C source code, to serve as a future reference.

Thanks to Hood Chatham, Siu Kwan Lam and Justin Wood for the feedback on drafts.

Claude code modified my .bashrc without asking?

Tue, 08 Jul 2025 09:59:40 +0000

Claude code modified my .bashrc without asking?

!!! note "Note" This is not the classical post about running claude in YOLO mode and then complaining that it damaged the system. I have reasons to think that claude automatically modified my .bashrc to remove a line alias claude=... without asking my permission and without notifying me of the change.

Tales with claude code: how to make it behave?

Tue, 27 May 2025 15:56:43 +0000

Tales with claude code: how to make it behave?

In the past weeks, I have been experimenting with using claude code to speedup development, in particular of SPy.

My experience so far reveals a clear pattern: claude excels at simple,one-shot tasks that follow existing patterns, producing commit-readycode. However, for complex tasks requiring multiple iterations, qualitydeteriorates significantly with each round, often necessitating completerewrites or extensive cleanup.

Because of this, I have been looking for ways to guide it towards making itonly simple steps, and to wait for my confirmation before going further. Sofar, I failed.

Over the clouds: CPython, Pyodide and SPy

Sun, 16 Mar 2025 18:06:02 +0000

Over the clouds: CPython, Pyodide and SPy

The Python community is awesome.

It is full of great people and minds, and interacting with people atconferences is always nice and stimulating. But one of my favorite things isthat over time, after many conferences, talks, pull requests and beers, thepersonal relationship with some of them strengthen and they become friends.

I am fortunate enough that two of them, Łukasz Langa and Hood Chatham, accepted myinvitation to join me in Cervinia, at the border between Italian and Swiss Alps,for a week of hacking, winter sports and going literally over the clouds. Thisis a brief summary of what we did during our time together.

Hello, HPy

Tue, 11 Feb 2025 13:49:52 +0000

!!! note "" Originally published on the HPy blog.

HPy has been around for a while now. The initial discussion started duringEuroPython 2019, in the good old times when we could still go to conferencesand have real-life meetings. Since then, HPy progressed a lot from thepoint of view of the actual code, but we have been a bit too silentw.r.t. communicating what we are doing to the external world and to the broaderPython community. Hopefully, now that this blog is online we will do a betterjob at periodically communicating the status of HPy, so make sure tosubscribe to the RSS feed.

hpy 0.0.2: First public release

Tue, 11 Feb 2025 13:49:52 +0000

!!! note "" Originally published on the HPy blog.

HPy 0.0.2 is out! This is the first version which is officially released andmade available on PyPI.

The major highlight of this release is that it is supported by three differentPython implementations: CPython, PyPy and GraalPython.

HPy @ Python Language Summit

Tue, 11 Feb 2025 13:49:52 +0000

!!! note "" Originally published on the HPy blog.

Yesterday I had the privilege to give a talk about HPy(sildes) at thePython Language Summit 2021.

The organizers of the summit will soon publish a full report about the event(edit: now availablehere),but for the HPy-specificpart,we got generally good feedback. Someone has a few concerns that if CPython isto change the API, HPy might not be going far enough. Others said that Pythonshouldn't wait for the "perfect" API if HPy can be the "good" one that helps itevolve.

Everyone was open to have HPy-compatible wheels on PyPI, once the HPyUniversal ABI stays relatively stable. Many people suggested that we shouldreally write a PEP to propose HPy as a "semi-official" API for Python.

Improve .NET Integration

Sat, 01 Jan 2000 00:00:00 +0000