Why does [[0] * 3] * 3 build a grid where setting one cell changes every row?

### What the `*` operator does to a list Sequence repetition on a list produces a **new list whose slots are filled with the same references** as the original. It never inspects, copies or recreates the elements — it copies pointers. That single sentence explains the whole puzzle, but the puzzle is only visible when the repeated element is mutable. Work through `[[0] * 3] * 3` as the interpreter does, inside out: 1. `[0] * 3` is evaluated **once**, yielding one list object — call it `row` — holding three references to the integer `0`. 2. `[row]` builds a one-element outer list holding one reference to `row`. 3. `* 3` builds a **new** three-slot list, and each of the three slots is filled with the same reference to `row`. So the expression creates exactly two list objects: one row, and one container that points at it three times. The row's reference count rises to three; no second or third row ever exists. ### Proving it rather than believing it Identity is the tool, because equality cannot see the difference — `[[0] * 3] * 3 == [[0] * 3 for _ in range(3)]` is `True`. Both compare equal element by element; only identity reveals that one of them is three views of a single object. ```pycon >>> grid = [[0] * 3] * 3 >>> grid[0] is grid[1] is grid[2] True >>> len({id(row) for row in grid}) 1 >>> good = [[0] * 3 for _ in range(3)] >>> len({id(row) for row in good}) 3 ``` A set of `id()` values collapsing to one is the cleanest demonstration: three slots, one object. (Comparing `id()` values is only meaningful while every object is still alive, which it is here because the grid holds them.) ### Why the write propagates `grid[0][0] = 1` is two operations: fetch the object in slot 0 of the outer list, then **mutate** that object's slot 0. The mutation lands on the one row that every outer slot references, so reading `grid[1][0]` afterwards reads the same memory and sees `1`. Nothing was copied and nothing was corrupted; the display simply prints one object three times. Contrast that with `grid[0] = [9, 9, 9]`, which does not mutate anything — it **rebinds** slot 0 of the outer list to a brand-new list. Afterwards row 0 is independent while rows 1 and 2 still share the original. That half-fixed state is a classic follow-up trap: the candidate patches one row, sees the symptom move, and concludes the grid is fine. ### Why the inner `[0] * 3` is safe The same operator, the same reference-copying behaviour — but integers are immutable. There is no operation that changes the object `0` into something else, so sharing it three times is unobservable. `row[0] = 1` rebinds a slot rather than mutating an integer, and touches only that row. The rule generalises: **repetition is safe exactly when the repeated element is immutable, or when you never mutate it.** `[None] * n` and `[0] * n` for pre-allocation are idiomatic and correct; `[[]] * n`, `[{}] * n` and `[SomeObject()] * n` are the trap, because the constructor or literal on the left runs once. ### The correct construction ```python grid = [[0] * 3 for _ in range(3)] ``` The difference is *when* the inner expression is evaluated. In the multiplication form it is evaluated once, before repetition; in the comprehension it is re-evaluated on every iteration of the loop, so each iteration produces a fresh object. For a variable-sized grid the same shape scales: `[[0] * cols for _ in range(rows)]`. If you already hold a list of rows and want independent ones, rebuild each row rather than copying the outer list — copying the outer list only duplicates the pointer array, and the duplicated pointers still address the same rows. ### How this shows up in review The smell to look for is `* n` applied to a list whose element is a mutable literal, a call, or a name bound to a mutable object. In a code review that pattern deserves a question every time: either the elements are immutable and it is fine, or the author wanted independent objects and has written a bug that will surface much later, as a mysterious "every record changed" report rather than as an exception.