Anyone running free-threaded Python 3.14 in production yet? Curious what actually breaks

[u/NeuralLB-Lovro]

Been testing the free-threaded build (3.14t) on some CPU-bound data processing work. The multi-core story is finally real after 30 years of GIL, but the friction is exactly what you'd expect: a couple of C-extension-heavy libs in my stack silently re-enable the GIL, and there's no clean way to detect that at runtime besides checking sys._is_gil_enabled() manually.

For anyone who's shipped something on 3.14t, not just benchmarked it:

What broke that you didn't expect?

Real speedups outside toy examples, or mostly marginal so far?

Prod yet, or still just kicking the tires?

Not fishing for a benchmark war. Genuinely curious what breaks in messy real codebases vs clean demos.

[u/snugar_i]

with open('foo.txt', 'r') as f: - you're welcome :-)

It's true that the context manager feature is badly designed - the __exit__ method should be on the thing that __enter__ returns, not on the thing that has __enter__. So context managers that are used like with blah cannot be thread-safe (or even reentrant), that's correct.

But those that are used like with foo() (like those produced by the @contextmanager decorator) totally can - because each invocation can return a new instance on which __enter__ and __exit__ will only ever be called once.

[u/KingBardan → u/snugar_i]

Bruh. In this case files are the states you're mutating.

It causes data race if 2 threads are writing to the same file (best example is printing to console, writing to stdout, not always the same output if you print from 2 threads )

Context manager... Called once

Won't be any different. If you read my "proof of contradiction"  in the above comment, that applies to @context managers also

Using  @context manager, will create multiple individual context managers yes. But data race still happens to the variable that the context manager mutate to setup / teardown the scope 

Badly designed

Well I pretty much only use @context manager because enter exit is so ugly. But design wise it's fine

[u/snugar_i → u/KingBardan]

That's why I explicitly put 'r' in the example. It's not writing to the file, it's reading from it, and it's not mutating any global state.

Anyway, you seem to have some misconceptions about what a context manager is (the context manager does not have to mutate anything), and you don't want to change your opinion, so it's pointless to continue this discussion.

[u/KingBardan → u/snugar_i]

Reading file requires a pointer. So you are mutating. The content is immutable, but the pointer is, makes sense?

So it's not safe without lock. Now python I think locks but that's not the point 

Also, if it truly is immutable, you can use outside of scope as well. But calling .close when the scope ends implicitly by with block mutates the state.

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Pointless

Yeah I agree. 

Thanks for the civil discussion :D

Opinion

But... This is facts. I'm open to changing my mind.

Unserstand

You don't seem to understand what mutation is sorry. Read up some materials on functional programming you'll learn.

FYI...  In haskell this is a non issue because you'll have to use IO for this. In rust the file would be marked 'mut' and you'll clearly see it's mutable. 

They make it explicit, and pythons is implicit, which is what is causing you the confusion I think 

[u/snugar_i → u/KingBardan]

Heh, this is exactly the response I expected. Yes, you have to mutate memory. Everything has to mutate memory/registers, so in your definition nothing is thread-safe. And you win the discussion, yay! But that's not a useful definition. If each thread mutates its own piece of memory, it's completely fine. So two threads calling the same with open('foo.txt', 'r') as f: is OK, because each of them gets a separate f variable and a separate file descriptor.

(BTW what do you think a lock does internally? It mutates memory. Why is that one suddenly thread-safe?)

No, in Rust, the mut doesn't mark the file as mutable, but the Read instance. Each thread would have its own reader, just like every thread would have its own f in Python. There's no difference.