Python 3.15 Brings Major JIT Compiler Upgrades, Still Disabled by Default

Python 3.15, with its first release candidate out on August 4, 2026, and a final release expected in October, delivers significant upgrades to CPython's experimental JIT compiler first introduced in Python 3.13. Key improvements include a new tracing frontend, basic register allocation, reduced reference-counting overhead, and in-place optimizations for integer and float operations. Early benchmarks show roughly 8–9% performance gains on x86-64 Linux and 12–13% on AArch64 macOS compared to the non-JIT interpreter. Despite the progress, the JIT remains disabled by default and is still considered experimental, with some interfaces explicitly marked unstable. The Python Steering Council paused new JIT development in June 2026 and requested a formal roadmap, leading to PEP 836, which proposes measurable targets including a 20% performance improvement by Python 3.17 — though the proposal remains under discussion.