What's New in Python 3.15

Python 3.15 is a feature-and-performance focused release that changes language, interpreter, C API, build options, and the standard library. Major language/implementation additions include explicit lazy imports (PEP 810) to cut startup time, a built-in immutable mapping frozendict (PEP 814), a sentinel builtin for unique marker objects (PEP 661), UTF‑8 as the default encoding (PEP 686), unpacking in comprehensions (PEP 798), and a stable ABI for free‑threaded builds plus frame pointers enabled by default for better observability. The standard library gains a dedicated profiling package (PEP 799) with Tachyon, an upgraded experimental JIT, more colorful CLI output, many module improvements, and a set of deprecations/removals and portability tweaks.

Key specifics: lazy imports use a new lazy soft keyword to create lightweight proxy names that load modules only on first use; global control is available via -X lazy_imports, PYTHON_LAZY_IMPORTS, and runtime APIs (sys.set_lazy_imports, sys.get_lazy_imports, sys.set_lazy_imports_filter), and types.LazyImportType lets code detect proxies. Lazy imports are restricted to module scope and exclude star/future imports; a lazy_modules fallback supports gradual adoption. Frozendict is an order‑preserving, hashable, non‑dict mapping accepted by many stdlib APIs and usable with eval/exec; sentinel gives concise, picklable unique sentinels. C API and build changes target robustness for free‑threaded interpreters and improved tooling integration.