On Tue, Dec 10, 2019 at 1:37 AM Andrew Barnert <abarn...@yahoo.com> wrote:
>
> On Dec 9, 2019, at 22:07, Wes Turner <wes.tur...@gmail.com> wrote:
> >
> >
> > Sets are not collections.abc.Sequence because they do not implement
> > __getitem__.
> > Are there other unordered Iterables in the standard library
>
> Again, it depends on what you’re trying to distinguish by that word
> “unordered”. Mappings (including dict) and their views, Sets (including
> frozenset), Iterators (including file objects, map, filter, generators, and
> most itertools functions), and many other things are Iterables without being
> Sequences. I have no idea which if any of these you mean by “unordered”. For
> every single one of them, just like for set, first(it) will give you the same
> value as list(it)[0] (albeit a different exception if they’re empty), so I
> don’t see why any of them are confusing. What else would you expect first to
> do? And, without knowing why you think first should mention any of them, I
> have no idea which ones it should mention.
(naievely) 'first' would seem to imply a stable ordering that doesn't
change with PYTHONHASHSEED='random' / -R (the default since 3.3)
The .first() docstring could mention that sets, in particular, may
have a different ordering for different interpreter invocations (when
the elements are integers less then sys.hash_info.modulus).
>
> >> On Tue, Dec 10, 2019 at 12:44 AM Tim Peters <tim.pet...@gmail.com> wrote:
> >>
> >> ... as is my _current_ idiom:
> >>
> >> for a in iterable:
> >> break
> >> else:
> >> raise ...
> >
> > Is this pattern in the tutorial?
>
> No, but you will find it on StackOverflow. I don’t think it needs to be in
> the tutorial. If needing to raise a specific exception—or just not
> StopIteration—is common enough that novices need to learn it, I think it’s
> common enough that it should be given a name and put in the itertools recipes
> or module, or at least pointed out in more-itertools, rather than teaching
> people to spell it out every time.
>
> >> Plain old
> >>
> >> a = first(iterable)
> >>
> >> would be perfect - but only if it raised.
> >
> > +1. How is this distinct from:
> >
> > first = next
>
> Because first works with any Iterable; next works only with iterators.
def first(iterable, *args):
return next(iter(iterable), *args)
first.__doc__ = (next.__doc__.replace('next(', 'first(') + """
.. note:: func:`next` takes a second argument as a default value
.. warning:: first(set('abc')) will only sometimes return 'a'""")
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