Negative subscripts

[Chris Angelico]

On Tue, Nov 30, 2021 at 6:45 AM dn via Python-list
<python-list at python.org> wrote:
>
>
>
> On 27/11/2021 21.23, Chris Angelico wrote:
> > On Sat, Nov 27, 2021 at 7:21 PM dn via Python-list
> > <python-list at python.org> wrote:
> >> The expression list is evaluated once; it should yield an iterable
> >> object. An iterator is created for the result of the expression_list.
> >> The suite is then executed once for each item provided by the iterator,
> >> in the order returned by the iterator. Each item in turn is assigned to
> >> the target list using the standard rules for assignments (see Assignment
> >> statements), and then the suite is executed. When the items are
> >> exhausted (which is immediately when the sequence is empty or an
> >> iterator raises a StopIteration exception), the suite in the else
> >> clause, if present, is executed, and the loop terminates.
> >> »
> >> https://docs.python.org/3/reference/compound_stmts.html#the-for-statement
> >>
> >>
> >> That said, I'm wondering if all is strictly true when the
> >> expression_list is a generator, which by definition features
> >> lazy-execution rather than up-front list production. So, things may
> >> depend upon the full-nature of the application.
> >
> > Yes, it is. Evaluating a generator expression gives you a generator
> > object, which is an iterable.
>
>
> You (and the text) are correct - the expression list is "evaluated once"
> and produces a generator object. For a particular understanding of
> "evaluate".
>
> However, as described, all that has been "evaluated" is a
> generator-object. Unlike (say) a list's iterator, a generator only
> eventually produces a 'list' - and each time the generator is
> called-upon to yield the next value, that value has to be "evaluated",
> ie there's some further evaluation loop-by-loop.
>
> Further, that the values-returned can be amended during the life of the
> generator (in ways anticipated and unanticipated by coder and
> interpreter alike). Thus, it seems that the "list" doesn't actually
> exist, as in, hasn't been "evaluated" as data-values, when the loop is
> enacted.
>
> What has been 'evaluated' are the terms by which the looping will
> proceed, and terminate.
>

That's true of ALL iterators though. If you get a list iterator, and
while you're stepping through it, the underlying list changes, you'll
see those changes. Nothing gets "snapshot" unless you explicitly
request it (eg by slicing the list first).

ChrisA