The order of iterable de-referencing in assignment?

[Marko Rauhamaa]

Shiyao Ma <i at introo.me>:

> Given a = [1, 2]
>
> a.extend(a) makes a = [1,2, 1,2]
>
> One might guess a.extend(a) would turn into an infinite loop. It turns
> out here Python first gets all the items of `a' and then append them
> to `a', so the infinite loop is avoided.

Functionally, Python's lists are not linked lists but, rather, vectors.

> My question is, is there any doc on the behavior of things like this?

No. It should be here:

   <URL: https://docs.python.org/3/reference/datamodel.html>

It only states:

   The items of a list are arbitrary Python objects.

and inexplicably muddies the waters by continuing:

   Lists are formed by placing a comma-separated list of expressions in
   square brackets. (Note that there are no special cases needed to form
   lists of length 0 or 1.)

(which has nothing whatsoever to do with the data model).


Marko