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Python 3.15's JIT is now back on track

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Re: Python 3.15's JIT is now back on track

#151

Post by bmitc »

mathisfun123 wrote: post #148
mattclarkdotnet wrote: post #132

Earlier quoted context omitted.

To clarify, it is nuts that in an object method, there is a performance enhancement through caching a member value. class SomeClass def init(self) self.x = 0 def SomeMethod(self) q = self.x ## do stuff with q, because otherwise you're dereferencing self.x all the damn time

> it is nuts that in an object method, there is a performance enhancement through caching a member value i don't understand what you think is nuts about this. it's an interpreted language and the word `self` is not special in any way (it's just convention - you can call the first param to a method anything you want). so there's no way for the interpreter/compiler/runtime to know you're accessing a field of the class…

I don't think it's a hot take to say much of Python's design is nuts. It's a very strange language.

Re: Python 3.15's JIT is now back on track

#152

Post by bmitc »

smcl wrote: post #117
owaislone wrote: post #68

Oh man, Python 2 > 3 was such a massive shift. Took almost half a decade if not more and yet it mainly changing superficial syntax stuff. They should have allowed ABIs to break and get these internal things done. Probably came up with a new, tighter API for integrating with other lower level languages so going forward Python internals can be changed more freely without breaking everything.

I cannot believe people are still acting like Python 2->3 was a huge fuck-up and an enormous missed opportunity. When in reality Python is by most measures the most popular language and became so AFTER that switch. Since the switch we have seen enormous companies being built from scratch. There is no reason for anyone to be complaining about it being too hard to upgrade in 2026

Those are unrelated.

Re: Python 3.15's JIT is now back on track

#153

Post by mattip »

cpgxiii wrote: post #102
rtpg wrote: post #83

Earlier quoted context omitted.

It's interesting you mention __del__ because Javascript not only doesn't have destructors but for security reasons (that are above my pay grade) but the spec _explicitly prohibits_ implementations from allowing visibility into garbage collection state, meaning that code cannot have any visibility into deallocations. I think __del__ is tricky though. In theory __del__ is not meant to be reliable. In practice CPython r…

> In practice CPython reliably calls it cuz it reference counts ... In a world where more people were using PyPy we could have pressure from that perspective to avoid leaning into it A big part of the problem is that much of the power of the Python ecosystem comes specifically from extensions/bindings written in languages with manual (C) or RAII/ref-counted (C++, Rust) memory management, and having predictable Python…

That cleanup can be explicit when needed by using context managers. Mixing resource handling with object lifetime is a bad design choice

Re: Python 3.15's JIT is now back on track

#154

Post by mattclarkdotnet »

mathisfun123 wrote: post #148
mattclarkdotnet wrote: post #132

Earlier quoted context omitted.

To clarify, it is nuts that in an object method, there is a performance enhancement through caching a member value. class SomeClass def init(self) self.x = 0 def SomeMethod(self) q = self.x ## do stuff with q, because otherwise you're dereferencing self.x all the damn time

> it is nuts that in an object method, there is a performance enhancement through caching a member value i don't understand what you think is nuts about this. it's an interpreted language and the word `self` is not special in any way (it's just convention - you can call the first param to a method anything you want). so there's no way for the interpreter/compiler/runtime to know you're accessing a field of the class…

What's nuts is that the language doesn't guarantee that successive references to the same member value within the same function body are stable. You can look it up once, go off and do something else, and look it up again and it's changed. It's dynamism taken to an unnecessary extreme. Nobody in the real world expects this behaviour. Making it just a bit less dynamic wouldn't change the fundamentals of the language but it would make it a lot more tractable.

Re: Python 3.15's JIT is now back on track

#155

Post by musicale »

mattclarkdotnet wrote: post #125

Python really needs to take the Typescript approach of "all valid Python4 is valid Python3". And then add value types so we can have int64 etc. And allow object refs to be frozen after instantiation to avoid the indirection tax. Sensible type-annotated python code could be so much faster if it didn't have to assume everything could change at any time. Most things don't change, and if they do they change on startup (e…

> Python really needs to take the Typescript approach of "all valid Python4 is valid Python3

Great idea, but I'm not convinced that they learned anything from the Python 2 to 3 transition, so I wouldn't hold my breath.

If you want a language system without contempt for backward compatibility, you're probably better off with Java/C++/JavaScript/etc. (though using JS libraries is like building on quicksand.) Bit of a shame since I want to like Python/Rust/Swift/other modern-ish languages, but it turns out that formal language specifications were actually a pretty good idea. API stability is another.

Re: Python 3.15's JIT is now back on track

#156

Post by sfink »

jonathanlydall wrote: post #147
rtpg wrote: post #83

Earlier quoted context omitted.

It's interesting you mention __del__ because Javascript not only doesn't have destructors but for security reasons (that are above my pay grade) but the spec _explicitly prohibits_ implementations from allowing visibility into garbage collection state, meaning that code cannot have any visibility into deallocations. I think __del__ is tricky though. In theory __del__ is not meant to be reliable. In practice CPython r…

> meaning that code cannot have any visibility into deallocations. This is more pedantry than a serious question. JavaScript has WeakReference, sure it'd be cumbersome and inefficient because you'd need to manually make and poll each thing you wanted to observe, but could it not be said that it does provide a view on deallocations?

Yes, WeakRef and FinalizationGroup both make GC visible (the latter removes the need to poll in your example). So not pedantic at all. They were eventually added after much reluctance from the language designers and implementers, partly because they can lead to code being broken by (valid & correct) engine optimizations, which is a big no-no on the web. But some things simply cannot be implemented without them.

Note that 90% of the uses for them actually shouldn't be using them, usually for subtle reasons. It's always a big cause for debate.

Re: Python 3.15's JIT is now back on track

#157

Post by sfink »

bastawhiz wrote: post #104
alpinisme wrote: post #99

Earlier quoted context omitted.

That link itself calls out that conformant implementations can’t be relied on to call callbacks. > A conforming JavaScript implementation, even one that does garbage collection, is not required to call cleanup callbacks. When and whether it does so is entirely down to the implementation of the JavaScript engine. When a registered object is reclaimed, any cleanup callbacks for it may be called then, or some time later…

It's supported in all of the major engines. And you also can't rely on the garbage collector to run at a predictable time (or at all!), so the engine never calling finalizers is functionally the same as the garbage collector being unusual.

The only (other) visible effect of GC not running is memory exhaustion. WeakRef/FinalizationGroup not getting triggered can have lots of script-visible effects, so can be much much worse. I wouldn't describe that as "functionally the same".

Re: Python 3.15's JIT is now back on track

#158

Post by sfink »

rtpg wrote: post #98
nvme0n1p1 wrote: post #86

Earlier quoted context omitted.

> code cannot have any visibility into deallocations Doesn't FinalizationRegistry let you do exactly that? https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

Oh! While this one does mention that you don't have visibility, this + weak refs seem to change the game I remember a couple of years ago (well probably around 2021) reading about GC exposure concerns and seeing some line in some TC39 doc like "users should not have visibility into collection" but if we've shipped weakrefs sounds like we're not thinking about that anymore

We still try to limit any additional exposure as much as possible, and WR/FG are specced to keep the visibility as coarse as possible. (Collections won't be visible until the current script execution finishes, though async adds a lot more places where that can happen.)

A proposal to add new ways of observing garbage collection will still be shot down immediately without a damn good justification.

Re: Python 3.15's JIT is now back on track

#159

Post by rtpg »

mattclarkdotnet wrote: post #154
mathisfun123 wrote: post #148

Earlier quoted context omitted.

> it is nuts that in an object method, there is a performance enhancement through caching a member value i don't understand what you think is nuts about this. it's an interpreted language and the word `self` is not special in any way (it's just convention - you can call the first param to a method anything you want). so there's no way for the interpreter/compiler/runtime to know you're accessing a field of the class…

What's nuts is that the language doesn't guarantee that successive references to the same member value within the same function body are stable. You can look it up once, go off and do something else, and look it up again and it's changed. It's dynamism taken to an unnecessary extreme. Nobody in the real world expects this behaviour. Making it just a bit less dynamic wouldn't change the fundamentals of the language bu…

In Python attribute access aren't stable! `self.x` where `x` is a property is not guaranteed to refer to the same thing.

And getting rid of descriptors would be a _fundamental change to the language_. An immeense one. Loads of features are built off of descriptors or descriptor-like things.

And what you're complaining about is also not true in Javascript world either... I believe you can build descriptor-like things in JS now as well.

_But_ if you want that you can use stuff like mypyc + annotations to get that for you. There are tools that let you get to where you want. Just not out of the box because Python isn't that language.

Remember, this is a scripting language, not a compiled language. Every optimization for things you talk about would be paid on program load (you have pyc stuff but still..)

Gotta show up with proof that what you're saying is verifiable and works well. Up until ~6 or 7 years ago CPython had a concept of being easy to onboard onto. Dataflow analyses make the codebase harder to deal with.

Having said all of that.... would be nice to just inline RPython-y code and have it all work nicely. I don't need it on everything and proving safety is probably non-trivial but I feel like we've got to be closer to doing this than in the past.

I ... think in theory the JIT can solve for that too. In theory

Re: Python 3.15's JIT is now back on track

#160

Post by fulafel »

mattclarkdotnet wrote: post #154
mathisfun123 wrote: post #148

Earlier quoted context omitted.

> it is nuts that in an object method, there is a performance enhancement through caching a member value i don't understand what you think is nuts about this. it's an interpreted language and the word `self` is not special in any way (it's just convention - you can call the first param to a method anything you want). so there's no way for the interpreter/compiler/runtime to know you're accessing a field of the class…

What's nuts is that the language doesn't guarantee that successive references to the same member value within the same function body are stable. You can look it up once, go off and do something else, and look it up again and it's changed. It's dynamism taken to an unnecessary extreme. Nobody in the real world expects this behaviour. Making it just a bit less dynamic wouldn't change the fundamentals of the language bu…

> Nobody in the real world expects this behaviour.

For example, numbers and strings are immutable objects in Python. If self.x is a number and its numeric value is changed by a method call, self.x will be a different object after that. I'd dare say people expect this to work.

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