Python 3.15 Release Highlights

[u/ossm-me]

Python 3.15 has reached Release Candidate 1, and the final release is expected on October 1, 2026.

I went through the changes and tried to summarize the ones that seem most relevant for everyday Python development without going through all the PEP numbers.

- Faster startup with lazy imports

Python 3.15 introduces lazy imports, which means some modules can be loaded only when they are actually needed instead of being loaded immediately.

This could help applications and CLI tools that spend a noticeable amount of time importing modules before doing any actual work.

- UTF-8 becomes the default

UTF-8 is becoming the default encoding, which should reduce encoding-related problems, especially when code is running on different operating systems.

This should make situations where something works on one computer but fails because of a different system encoding less common.

- Built-in immutable dictionaries

Python 3.15 adds a built-in "frozendict"-style immutable mapping.

Similar to how a tuple provides an immutable alternative to a list, this gives Python developers a standard way to work with dictionaries that cannot be modified.

- JIT improvements

The JIT compiler continues to improve in Python 3.15.

Early benchmarks show performance improvements in some workloads, including roughly 8–9% on Linux and larger improvements in some Apple Silicon tests.

These numbers will obviously depend on the workload, so I would wait for more benchmarks before making broader conclusions.

- New profiler: Tachyon

Python 3.15 also introduces Tachyon, a new sampling profiler designed for very low overhead.

It can sample running programs at very high frequencies and can also be attached to an already-running process.

This could be useful for finding performance problems without having to restart an application with a profiler attached from the beginning.

- Some terminal improvements

The interactive Python experience is getting a few smaller improvements, including colored error messages and prompts.

The "sqlite3" command-line interface also gets SQL keyword completion.

- Free-threaded Python is still opt-in

Python 3.15 does not make the no-GIL/free-threaded build the default.

It is still something you have to explicitly use.

However, free-threaded Python is becoming more mature, including improvements to ABI support that should make it easier for C extension developers to support it.

Overall, Python 3.15 doesn't look like a release that completely changes how Python is written. Most of the changes are focused on performance, tooling, and improving some long-standing parts of the language.

Since this is already RC1, the major feature set should be mostly locked and the remaining work should mainly be bug fixes and final polishing.

Has anyone here been testing the Python 3.15 beta or RC?

I'm especially interested in whether lazy imports have caused compatibility problems with existing projects.

[u/Significant_Map_19]

lazy imports are going to break so many frameworks that do weird metaprogramming stuff at startup, I can already feel the bug reports piling up

the frozendict thing is nice though, been using dicts as immutable keys with wrappers for years

[u/SCD_minecraft → u/Significant_Map_19]

It's opt in

import foo # nothing changes, just as before

lazy import bar # acually loaded only when needed

[u/Rodot → u/SCD_minecraft]

This does make me wonder if lazy may at some point become more general. Could we one day see things like the following?

```

lazy result_a = f(x)

lazy result_b = g(y)

if x > y:

    return result_a

elif x > result_b:

    return result_b

```

[u/ProtectionOne9478 → u/Rodot]

You can do this now with async.

[u/zurtex → u/ProtectionOne9478]

Or just lambdas, if you don't mind recalculating:

result_a = lambda: f(x)
result_b = lambda: g(y)

if x > y:
    return result_a()
elif x > result_b():
    return result_b()

Or cache and partial if you do mind reclalculating:

from functools import cache, partial

result_a = cache(partial(f, x))
result_b = cache(partial(g, y))

if x > y:
    return result_a()
elif x > result_b():
    return result_b()

People see the word "lazy" and forget that's what a function basically is.

[u/ProtectionOne9478 → u/zurtex]

yep, those work too. i use async a lot so it was just the first i thought of.

partial is probably the best way, since you don't have to be in an async context and changes to x won't change the behavior of the function like it will for the lambda, eg you'll get 2 for a() in the following code which could lead to unexpected behavior:

x = 1
a = lambda: x
x = 2
a()

[u/RingularCirc → u/zurtex]

A generic (and perfectly-typable) Lazy[T] class is also easy to write once and for all (though for a thread-safe one it'll require adding locking and stuff). Something like:

class Lazy[T]:
    def __init__(self, computation: Callable[[], T]) -> None:
        self._f: Callable[[], T] | None = computation
        self._val: T | None = None
    @property
    def value(self) -> T:
        if self._f:
            self._val = self._f()
            self._f = None
        return self._val

I hope it's correct (I've written it once but can't find quickly enough); bubbling up an exception from _f() when lazy.value is accessed is the intended behavior, no need to stow it away as some version of caching do because we won't get to erasing _f in this case and will try the computation once more later... which... well, I'm not sure now but it's no worse than second best choice anyway.

Oh yeah typecheckers will still be mad at this code, most probably. But IMO in such a small self-contained class we can safely use # type: ignore at return. There's a good solution to typing here if Lazy stores a _x: C | V where C holds the callable, V holds the value and both are provably disjoint, for example C = tuple[Literal[True], Callable[[], T]] and V = tuple[Literal[False], T]. Even not as cumbersome to annotate as I expected (and the code is a simple if self._x[0]: ....

EDIT: For convenience this can also accept arguments for the callable in __init__ and pack it all in a partial, yeah, to allow the caller not to bother.

[u/SCD_minecraft → u/Rodot]

Questionable

imports aren't really ment to have side effects, while function have side effects quite often

[u/Brian → u/Rodot]

I remember early versions of pypy had an experimental feature like that, where you could have a thunk object space where access to the value would trigger it to become the result of the evaluation. I kind of doubt it'll ever be added to python though - laziness allows some cool stuff, but there's lots of potential for bugs and weirdness with it.

[u/hotsauce56 → u/Rodot]

They talked about it a bit on Core.py podcast. There’s technically already plumbing for it with the lambda keyword

[u/jdehesa → u/Significant_Map_19]

You have to explicitly opt in for lazy imports, it's not going to effect existing code.

[u/billsil → u/jdehesa]

It’s not going to be in existing code for 5+ years. It’s going to break compatibility with old versions unless they retroactively add a future import.

[u/chase45424 → u/billsil]

It's isn't a future import but there is a backwards compatible way to specify an import as lazy (similar to dunder all) that will be a no-op on older python versions.

[u/HommeMusical → u/billsil]

Existing code will not be broken.

Lazy imports will never be the default.

There will be an environment variable you can set to make imports lazy everywhere, but that's strictly optional.

[u/Wonderful-Habit-139 → u/HommeMusical]

They will be default once Python versions that don't support it reach EOL.

[u/HommeMusical → u/Wonderful-Habit-139]

Your statement is false.

The PEP says in multiple cases that there is no plan to do this, e.g. https://peps.python.org/pep-0810/#module-level-lazy-import-mode and https://peps.python.org/pep-0810/#making-the-new-behavior-the-default

[u/Wonderful-Habit-139 → u/HommeMusical]

Ahh you mean like actually the default.

I meant more like, using lazy imports instead of workarounds afterwards. I focused more on the "existing code will not be broken" part it seems.

Guess we agree then.

[u/billsil → u/HommeMusical]

An environment variable isn’t going to make python 3.14 work with lazy imports.

[u/HommeMusical → u/billsil]

I'm not seeing your point.

It has always been the case that new features in the language cannot work with older versions of the language, so you pick the minimum version you support, and use that feature set.

[u/wRAR_ → u/billsil]

Which is also not a breakage.

[u/billsil → u/wRAR_]

It is if you want to support multiple versions of python.

Backwards compatibility should not be the only goal. Forwards compatibility matters as well.

[u/wRAR_ → u/billsil]

You seem confused.

[u/wRAR_ → u/billsil]

I expect __lazy_modules__ to be used in some amount, but also not all "existing code" is OSS libraries and apps that are expected to run on all supported Python versions, you can freely use it in your local code.

[u/billsil → u/wRAR_]

I run an open source project, so I’m acutely aware of dependencies. On every supported python version, you should support every version of a dependency (assuming your dependencies don’t conflict). I code work libraries in the same way with a reduced set of python versions.

It’s not up to me to specify a python version. It’s a per project/team decision.

[u/wRAR_ → u/billsil]

Well if you run an open source project then yes, you should fall back to __lazy_modules__.

[u/wRAR_ → u/Significant_Map_19]

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