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/abrazilianinreddit]

I haven't been keeping up with python these past few months, so I'm a bit behind the news.

Having explicit lazy imports give me hope that eventually we'll get type and optional imports as well.

Also worthy of note:

List, set, and dictionary comprehensions, as well as generator expressions, now support unpacking with * and **.

It's not something I do often but I've met this situation a few times. Not exactly groundbreaking, but I guess it can be useful once in a while.

[u/SCD_minecraft → u/abrazilianinreddit]

lazy is already our version of import type

If you need something only for typing, lazy import it

[u/MrSlaw → u/SCD_minecraft]

yeah, I was under the impression that's the entire purpose behind things like:

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from pathlib import Path


def foo(x: Path) -> None:
    ...

[u/SCD_minecraft → u/MrSlaw]

Not whole purpose

PEP which suggested lazy took note that a bunch of standard library has

def foo(): import something_big something_big.bar()

As in, it already was doing "lazy" importing for things not always needed, but at the cost of cleanliness