Python JIT Compiler (Copy and Patch) — ELI5

Imagine you’re translating a book from English to Spanish. You could translate word by word every time someone asks for a page — that’s slow. Or you could make pre-printed cards with common sentences and just fill in the blanks (names, dates, numbers). Way faster.

That’s exactly what Python’s new JIT compiler does.

What is a JIT? JIT stands for “Just In Time.” Normally, Python reads your code one instruction at a time and figures out what to do — kind of like a human reading a recipe step by step. A JIT compiler watches which steps you repeat a lot and translates them into your computer’s native language (machine code) so the computer can execute them directly, without the middleman.

The “copy and patch” trick. Most JIT compilers are incredibly complex — they’re like building a whole new translation engine inside Python. Python’s approach is simpler. During development, each Python instruction gets pre-translated into a machine code template (a “stencil”). At runtime, Python just copies the right stencil and patches in the specific details — like filling in the blanks on those pre-printed cards.

Is it fast? Right now, the speedup is modest — maybe 5-10% on some programs. But this is just the beginning. The important thing is that the architecture is in place. Future Python versions will make the JIT smarter, like adding shortcuts for common patterns.

Do you need to do anything? Nope. If the JIT is available and enabled, it works automatically. You don’t change your code at all.

The one thing to remember: Python’s copy-and-patch JIT is like pre-made sentence cards for a translator — simple, practical, and a foundation for making Python much faster in the future.