Python 3.15.0 Stable Release Ships With Lazy Imports, frozendict and sentinel Built-ins, and UTF-8 as the Default Encoding
Python.org lists 3.15.0 as the stable release dated Oct. 9, 2026, with explicit lazy imports, new frozendict and sentinel built-in types, and UTF-8 as the default encoding.
Overview
The Python.org download page lists Python 3.15.0 as a stable release dated Oct. 9, 2026, describing it as containing “many new features and optimisations compared to Python 3.14, in 5,643 commits from 1,012 contributors.” The release schedule in PEP 790 lists “3.15.0 final: Friday, 2026-10-09”, so the release is dated on its scheduled day.
The Machine Herald previously reported on the release-candidate phase. This article covers what the final release page and the official What’s New document say.
What We Know
Language and interpreter
The release page lists six interpreter-level changes: PEP 661 adds a sentinel built-in type, PEP 686 makes UTF-8 the default encoding, PEP 798 allows unpacking in comprehensions, PEP 810 adds explicit lazy imports, PEP 814 adds a frozendict built-in type, and PEP 829 adds package startup configuration files.
According to the What’s New in Python 3.15 document, lazy imports use a new lazy soft keyword: when an import is marked lazy, “Python defers the actual module loading until the imported name is first used.” The document says the keyword works with both import and from ... import statements, and that the -X lazy_imports command-line option and the PYTHON_LAZY_IMPORTS environment variable accept the values all and normal. It also states that lazy imports are only permitted at module scope, and that using lazy inside a function, class body, or try/except/finally block raises a SyntaxError.
The same document says the new frozendict type “does not allow modification after creation,” is not a subclass of dict, and is hashable “as long as all of its keys and values are hashable.” It lists copy, decimal, json, marshal, plistlib (only for serialization), pickle, pprint and xml.etree.ElementTree among the standard library modules updated to accept it.
The sentinel type is described in What’s New as a way of “creating unique sentinel values with a concise representation.” The document credits Jelle Zijlstra for the contribution and Tal Einat for writing PEP 661.
On encoding, What’s New says Python “now uses UTF-8 as the default encoding, independent of the system’s environment,” which applies only when no encoding argument is given. To retain the previous behaviour, the document points to the PYTHONUTF8=0 environment variable or the -X utf8=0 command-line option.
InfoWorld’s overview of the release frames the encoding change as meaning developers no longer need to specify UTF-8 when, for instance, reading text from files.
Profiling, JIT and build changes
The release page lists a dedicated profiling package under PEP 799 and “Tachyon: High frequency statistical sampling profiler.” What’s New says the sampler is exposed as profiling.sampling, that profiling.tracing is relocated from cProfile, and that cProfile “remains as an alias for backwards compatibility.” The profile module is deprecated and, according to the document, will be removed in Python 3.17.
On the experimental JIT compiler, the release page reports “7-8% geometric mean performance improvement on x86-64 Linux over the standard interpreter, and 11-12% speedup on AArch64 macOS over the tail-calling interpreter.” The earlier Machine Herald article cited release-candidate figures of roughly 8-9 percent on x86-64 Linux and 12-13 percent on AArch64 macOS, so the final-release page’s figures are about one percentage point lower. InfoWorld separately reports “an 8% to 13% geometric mean performance improvement over standard CPython, depending on the platform and workload.”
PEP 831 enables frame pointers by default. According to What’s New, CPython is now built with the compiler flags -fno-omit-frame-pointer and -mno-omit-leaf-frame-pointer on platforms that support them, and builds can opt out with --without-frame-pointers.
The release page also notes two packaging changes: official Windows 64-bit binaries now use the tail-calling interpreter, and official macOS binaries now install free-threading support by default.
What We Don’t Know
The release page carries a notice for macOS users: an operating system behavior change in macOS 27.0 may cause IDLE and other tkinter applications to hang when a menu command opens a dialog. The page says the problem is “believed to affect all current versions of the Tk graphics toolkit” and points to issue #158053 for updates. No workaround is described on the page.
The sources reviewed do not report adoption figures for 3.15 or how quickly downstream distributions and package maintainers plan to move to it. The JIT is described on the release page as experimental, and the sources do not say when it might become a default.
Why It Matters
The release page puts several changes with direct effects on existing code in the same version: the UTF-8 default changes the behaviour of open() calls that omit an encoding, as What’s New’s own example open('flying-circus.txt') illustrates. For code that must also support versions older than 3.15, What’s New states that a module can define __lazy_modules__ as a container of fully qualified module name strings so that regular import statements for those modules are treated as lazy.