[Python-ideas] Consider making enumerate a sequence if its argument is a sequence

[Steven D'Aprano]

On Thu, Oct 01, 2015 at 04:13:37PM -0700, Chris Barker wrote:
> On Thu, Oct 1, 2015 at 12:37 PM, Steven D'Aprano <steve at pearwood.info>
> wrote:
> 
> > This makes a difference. Just a few days ago, somebody mis-reported a
> > supposed "bug" in all() and any(). For example:
> >
> > values = (x%5 == 3 for x in range(8))
> > print(list(values))
> > print(all(values))  # should return False
> >
> > Obvious error is obvious: having printed out the values from the
> > generator expression, values is now exhausted, and all() of the empty
> > set is True (vacuous truth). The difference between general iterables
> > which may or may not be one-shot iterators, and those which are
> > definitely not iterators, is not always just an implementation detail.
> 
> 
> they used a generator expression, when they clearly wanted a list
> comprehension -- so yes, it matters what they were getting, I don't know
> that adding more vocabulary would help prevent people from making that
> mistake...
> 
> if they had been smart enough to call the list() again, before claiming
> there was a bug in all -- it may have explained itself.

You're missing the point. Don't focus on the fact that the bug was in 
their understanding of what their code did. Let's just pretend that 
their *intentional* algorithm was:

def alg(iterable):
    print(list(iterable))
    print(all(iterable))

and for the sake of bring this never-ending thread to an end, let's 
agree for the sake of argument that it cannot be re-written in any other 
way.

Since the semantics of the function are intentional and correct, the 
parameter is named misleadingly. *iterable* is not sufficiently precise, 
because the function does not accept any old iterable -- it fails to 
work correctly on *iterators*, are a sub-kind of iterable.

If you want a more practical example, any algorithm which needs to 
iterate over an interable two or more times needs to specify "iterable 
which is not an iterator".


-- 
Steve