On Fri, Apr 26, 2019 at 12:17:57AM -0400, Terry Reedy wrote:
> On 4/25/2019 7:12 PM, Greg Ewing wrote:
> >Steven D'Aprano wrote:
> >>I too often forget that reverse() returns an iterator,
>
> I presume you mean reversed(). list.reverse() is a list
Yes, I meant reversed(), not list.reverse() which is an in-place mutator
method and returns None.
> >That seems like a mistake. Shouldn't it return a view?
>
> RL = reversed(somelist) is already partly view-like. The nth next call
> returns the nth item at the time of the next call, rather than at the
> time of the reversed call. However, the number of items produced by
> next calls is the length of the list at the time of the reversed call.
That's not quite correct:
py> L = [1, 2, 3]
py> R = reversed(L)
py> L.clear()
py> next(R)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration
It seems that:
- in-place modifications in the list are reflected in the items
yielded (so reversed() doesn't make a copy of the list);
- operations which extend the length of the list don't show up
in the reversed version;
- and operations which decrease the length of the list decrease
the number of items yielded.
That suggests to me an implementation similar to:
# untested
def reversed(alist):
N = len(alist)
for i in range(N-1, -1, -1):
try:
yield alist[i]
except IndexError:
break
raise StopIteration
which I suppose is close to what you meant here:
> The first next(RL) is the current somelist[captured_length].
--
Steven
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