CPython 3.15

Definition

CPython 3.15 is the October 9, 2026 stable release of the reference Python implementation, bundling performance, encoding, typing, and runtime packaging changes.

Key Points

• JIT: Experimental JIT upgraded — 7–8% geometric mean speedup on x86-64 Linux vs standard interpreter; 11–12% on AArch64 macOS vs tail-calling interpreter (2026-10-09-python-315-insider-blog)
• PEP 686: UTF-8 default for open() and stdio without explicit encoding; use encoding="locale" or PYTHONUTF8=0 to revert
• PEP 810 lazy imports, PEP 814 frozendict, PEP 661 sentinel, PEP 829 package startup config
• Official macOS binaries ship free-threading by default; Windows 64-bit uses tail-calling interpreter
• Known issue: macOS 27.0 tkinter/IDLE hang (2026-10-09-python-3-15-0-release)

Related

• python-tooling
• python-developer-tooling
• python-workers

Sources

• 2026-10-09-python-3-15-0-release
• 2026-10-09-python-315-insider-blog