Python 3.15 Released With Lazy Imports, UTF-8 by Default, and a Faster JIT

Python 3.15 Released With Lazy Imports, UTF-8 by Default, and a Faster JIT

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Python 3.15.0 was released on October 9, about a week later than planned after a third release candidate.

Lazy imports

PEP 810 adds an opt-in lazy keyword:

lazy import json

The name is bound immediately, but the module is only loaded the first time you use it. Command-line tools that import large libraries they rarely need can start noticeably faster. Existing code is unaffected unless it opts in.

UTF-8 by default

PEP 686 makes UTF-8 the default text encoding. Opening a file without an encoding= argument no longer depends on the system locale. On Linux, where UTF-8 locales are already the norm, little changes; the big difference is on Windows. Code that relied on a non-UTF-8 locale encoding should be tested.

Faster JIT

The experimental JIT is now delivering real gains: around 7-8% faster than the standard interpreter on x86-64 Linux (geometric mean) and 11-12% on AArch64 macOS against the tail-calling interpreter.

Other additions

  • New built-in types: frozendict, an immutable, hashable dictionary, and sentinel, a standard way to create unique marker values
  • A new profiling package that includes Tachyon, a sampling profiler that attaches to running processes and can produce flame graphs
  • Frame pointers enabled by default, which helps native stack traces and tools like perf and eBPF
  • Stable ABI support for free-threaded builds
  • Better error messages and more colourised command-line output

Getting it

Distributions will not switch their system Python mid-release; Fedora and rolling distributions adopt it first. To try it now without touching the system interpreter, use a version manager such as uv or pyenv, or build it into /usr/local with make altinstall as our building from source guide describes.

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Background reading

Explainers for the concepts behind this story.

Debugging Bash Scripts: set -x, PS4, and ShellCheckHow to find out what a misbehaving shell script is actually doing: syntax checks with bash -n, tracing with set -x and a better PS4, tracing to a file, catching errors with trap, and finding bugs before they run with ShellCheck.getopts Explained: Parsing Script Arguments Without RegretReading positional parameters works until someone passes flags in a different order. getopts is built into bash and handles short options properly, getopt handles long ones, and there is a reason to avoid one of them.Shell Completion: How It Works and How to Write Your OwnWhy Tab knows your command options, where completion scripts live, and writing one for your own tool in bash and zsh.