On Sun, Dec 08, 2019 at 01:45:08PM +0000, Oscar Benjamin wrote:
> On Sat, 7 Dec 2019 at 00:43, Steven D'Aprano <st...@pearwood.info> wrote:
[...]
> > But there's a major difference in behaviour depending on your input, and
> > one which is surely going to lead to bugs from people who didn't realise
> > that iterator arguments and iterable arguments will behave differently:
> >
> > # non-iterator iterable
> > py> obj = [1, 2, 3, 4]
> > py> [first(obj) for __ in range(5)]
> > [1, 1, 1, 1, 1]
> >
> > # iterator
> > py> obj = iter([1, 2, 3, 4])
> > py> [first(obj) for __ in range(5)]
> > [1, 2, 3, 4, None]
> >
> > We could document the difference in behaviour, but it will still bite
> > people and surprise them.
>
> This kind of confusion can come with iterators and iterables all the
> time.
Do you have some concrete examples of where this is common, because I
don't recall seeing anything like this ever. Since next() doesn't accept
a non-iterator, this confusion doesn't come up for next. I suppose it
could come up with itertools islice:
py> s = "abcdefghijklmn"
py> [list(itertools.islice(s, 0, 1)) for __ in range(5)]
[['a'], ['a'], ['a'], ['a'], ['a']]
but I've never seen that in real code, so I wouldn't say it happens all
the time, or even a lot of the time. YMMV I guess, but I like to think
I have a reasonable grasp of the kinds of gotchas people often trip
over, and this isn't one of them.
The closest I can think of is the perennial gotcha that you can
iterate over a sequence as often as you like, but an iterator only once.
> I can see that the name "first" is potentially confusing.
> Another possible name is "take" which might make more sense in the
> context of partially consumed iterators.
There's already a take() in the itertools recipes. Rather than add a new
"first" itertools function, I'd rather promote `take` out of the recipes
and give it an optional default, something like this:
def take(n, iterable, /, *default):
if default:
(default,) = default
iterable = chain(iterable, repeat(default))
return list(islice(iterable, n))
"Get the first item" with a default then becomes:
a = take(1, iterable, default)[0]
"Get the first 5 items" becomes:
a, b, c, d, e = take(5, iterable, default)
If you want to distinguish the case where the iterable is empty or
shorter than you expect, you can pass a known sentinel and check for
that, or pass no default at all and test the length of the resulting
list.
--
Steven
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