[Python-ideas] Range and slice syntax

[Nicholas Harrison]

Overall, I agree with you. It is more intuitive to an experienced Python
user, and not so helpful to beginners. It decreases the ability to read out
code like English sentences and makes it harder to know what to search for
online. So it boosts facility after you know the language, but not when
starting out.

On Sun, Nov 11, 2018 at 2:35 AM Steven D'Aprano <steve at pearwood.info> wrote:

> On Sat, Nov 10, 2018 at 10:58:02PM -0700, Nicholas Harrison wrote:
>
> [...]
> > (start:stop:step)
> >
> >
> > Meet a range/slice object. Parentheses are required. (Its syntax in this
> > regard follows exactly the same rules as a generator expression.) I say
> > both range and slice because it can be used in either role.
>
> Ranges and slices are conceptually different things. Even if they have
> similar features, they are very different:
>
> - range is a lazy sequence, which produces its values on demand;
>
> - it supports (all? most?) of the Sequence ABC, including
>   membership testing and len();
>
> - but its values are intentionally limited to integers;
>
> - slice objects, on the other hand, are an abstraction referring
>   to a context-sensitive sequence of abstract indices;
>
> - those indices can be anything you like:
>
> py> s = slice("Surprise!", range(1, 100, 3), slice(None))
> py> s.start
> 'Surprise!'
> py> s.stop
> range(1, 100, 3)
> py> s.step
> slice(None, None, None)
>
>
> - they don't support membership testing, len() or other Sequence
>   operations;
>
> - most importantly, because they are context-sensitive, we don't
>   even know how many indexes are included in a slice until we know
>   what we're slicing.
>
> That last item is why slice objects have an indices() method that takes
> a mandatory length parameter.
>
> If slices were limited to single integer indices, then there would be an
> argument that they are redundant and we could use range objects in their
> place; but they aren't.
>
>
> [...]
> > Why is it useful? I at least find its syntax to be simple, intuitive, and
> > concise -- more so than the range(...) or slice(...) alternatives.
>
> Concise, I will grant, but *intuitive*?
>
> I have never forgot the first time I saw Python code, and after being
> told over and over again how "intuitive" it was I was utterly confused
> by these mysterious list[:] and list[1:] and similar expressions. I had
> no idea what they were or what they were supposed to do. I didn't even
> have a name I could put to them.
>
> At least range(x) was something I could *name* and ask sensible
> questions about. I didn't even have a name for this strange
> square-bracket and colon syntax, and no context for understanding what
> it did. There's surely few things in Python more cryptic than
>
>     mylist = mylist[:]
>
> until you've learned what slicing does and how it operates.
>
> Your proposal has the same disadvantages: it is cryptic punctuation that
> is meaningless until the reader has learned what it means, without even
> an obvious name they can refer to.
>
> Don't get me wrong: slice syntax is great, *once you have learned it*.
> But it is a million miles from intuitive. If this proposal is a winner,
> it won't be because it will make Python easier for beginners.
>
>
>
> [...]
> > sum(1:6) # instead of sum(range(1, 6))
>
> That looks like you tried to take a slice of a sequence called "sum" but
> messed up the brackets, using round instead of square.
>
>
> > list(1:6)
>
> Same.
>
> > for i in (1:6):
>
> Looks like a tuple done wrong.
>
>
> I think this is not an improvement, unless you're trying to minimize the
> number of characters in an expression.
>
>
> > It also makes forming reusable slices clearer and easier:
> >
> > my_slice = (:6:2) # instead of slice(None, 6, 2)
>
> "Easier" in the sense of "fewer characters to type", but "clearer"? I
> don't think so.
>
>
> [...]
> > So here's the part that requires a little more thought.
>
> Are you saying that so far you haven't put any thought into this
> proposal?
>
> *wink*
>
> (You don't have to answer this part, it was just my feeble attempt at
> humour.)
>
>
>
>
> --
> Steve
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