> The problem is that first() does not return the first item of the
> iterator, but the *next item still available*.
>
> a = list('12345')
> b = iter('12345')
>
> first(a) == first(a) # True
> first(b) == first(b) # False
This is an excellent point, and something I hadn't considered. Unless someone
can think of a good workaround that doesn't make the implementation hideously
complex, I retract my support for adding `first()` and `last()` to itertools.
> On 10 Oct 2021, at 05:09, Steven D'Aprano <[email protected]> wrote:
>
> On Wed, Oct 06, 2021 at 03:42:28PM +0100, Alex Waygood wrote:
>
>>> Whether they are added to dict or itertools, there are still nine of
>>> them
>>
>> No, the suggestion was to add two functions to itertools (first() and
>> last(), which would work with any iterable, not just dicts), rather
>> than adding nine methods to the dict interface. This was precisely why
>> I was saying that I liked the itertools solution more.
>
> Okay.
>
> In another post, I've explained why I don't think that putting first()
> in itertools would actually be useful (TL;DR: newbies wouldn't know it
> was there, and to people experienced enough to know it was there, it's
> likely easier to just use next() directly).
>
> When I started writing this post, I was going to argue that we should
> put first() and last() in itertools as recipes, for their pedagogical
> value:
>
> def first(iterable):
> return next(iter(iterable))
>
> def last(iterable):
> return next(reversed(iterable))
>
> I had got so far as to open bugs.python.org to create a ticket.
>
> But I've convinced myself that they aren't even worthwhile as recipes.
> The problem is that if we are being precise and accurate, and we should
> be, the names are simply *wrong* and the behaviour will be confusing to
> those we're supposedly trying to help.
>
> The problem is that first() does not return the first item of the
> iterator, but the *next item still available*.
>
> a = list('12345')
> b = iter('12345')
>
> first(a) == first(a) # True
> first(b) == first(b) # False
>
> If that behaviour makes sense to you, and is what you expected, then
> congratulations, you're probably a very sophisticated Pythonista who
> understands that iterators mutate after each item is retrieved, in
> which case you probably aren't going to get any benefit at all from a
> named first() function.
>
> But if you are one of those newbies who (we're told) need a named
> function, then you will probably be totally gobsmacked that sometimes
> first() will return the first item, and always the first item, and
> sometimes it will return the first, second, third... items in sequence.
>
> If we are to be precise and accurate, *sequences* have a first item, but
> iterators only have a next item.
>
>
>
> --
> Steve
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