Hey Reader,

Quick test. In this Python, where do numbers and `[1, 2, 3] live?

You probably know what it prints. Fewer Python devs can draw why:

Python never makes you draw this, because the runtime automatically tracks how many names point at that list and frees it when the count hits zero.

Now the equivalent example in Rust:

A `Vec` is a small fixed-size handle on the stack pointing at the real data on the heap.

Same for `String`.

So what happens here?

Rust copied the three-field handle and invalidated `numbers`.

Nothing was freed, nothing was copied on the heap.

In a recent Rust demo I admittedly said "A is freed up" after a move while teaching. This is the picture I should have had in my head.

And it's why Rust needs no garbage collector: there is exactly one owner, so the compiler knows the exact line where `[1, 2, 3] can be freed.

Python lets you ignore the question "who owns this?"

Rust requires you to answer it.

Without the picture, the rules feel like arbitrary compiler pedantry.

With it, they're just the compiler asking you to be explicit about something Python always did for you, but came with a hit to performance and sometimes safety.

(This is a mental model, not a full account of CPython or Rust internals. It's the model that makes the error messages start making sense.)

But here's also where it gets harder.

Moving ownership around works until you need two parts of your code to look at the same data.

In Python that's free. In Rust, that's called borrowing, and it's where most Python devs start fighting the compiler (no worries, in our cohort, they come out as best friends 🤝).

In week 3 of our cohort we dive deep into ownership, borrowing, and deciding between `String` and `&str, all in the context of building a real JSON parser from scratch.

You get a challenging project (good in AI times where we delegate more and more!), an engaging group, each PR reviewed by expert devs.

Learn new mental models. Learn an increasingly popular systems language.

Write better Python as a nice side-effect.

The next cohort starts in November → https://scriptertorust.com​

Cheers,

Bob & Jim

P.S. In the next email we’ll talk more about borrowing.

What `& actually points at, and why it can't outlive the owner.