Values and names

A variable is a name pointing at a value, not a box that remembers everything you ever put in it. Rebind the name and the old value is gone — unless you saved it somewhere first. Step through both versions and watch the names move.

price starts at 5, then becomes 7. Add the original price and the new price together (we want 5 + 7 = 12).

Reusing a name — the old value is gone execution-derived · CPython

variables

state after line 1 runs

variable	value
`price`	`5`

step 1 / 3

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What you are looking at

Keep your eye on price: one name, one value at a time.

Buggy — price = 5 then price = 7 — the second line doesn't add a value, it replaces the one the name points at. So answer = price + price is 7 + 7 = 14, never 5 + 7. The original 5 was never kept.
Procedural — Copy the value into old before reassigning price. Now both 5 and 7 survive under different names, and answer is the 12 we wanted.

Every state you see came from running the program under CPython's tracer at build time — see how GlassBox stays honest.