I like this idea of giving `slice` a metaclass that defines a `.__getitem__()` allowing us to construct slices on the slice type itself.

FWIW, this is exactly what pandas.IndexSlice does.  E.g., from http://pandas.pydata.org/pandas-docs/stable/advanced.html:
In [51]: dfmi.loc[(slice('A1','A3'),slice(None), ['C1','C3']),:]
In [52]: idx = pd.IndexSlice

In [53]: dfmi.loc[idx[:,:,['C1','C3']],idx[:,'foo']]
This is one of those nifty things that's buried in Pandas but not well documented.  I'd rather spell the above simply as:

dfmi.loc[slice[:,:,['C1','C3']], slice[:,'foo']]

I like the change proposed to `str(slice(10))` also... and it would be way better if `slice[:10]` were actual "syntax."  In fact, in that case it could even be the repr().

Note: Notwithstanding my scare quotes, Steven isn't actually asking for new syntax.  "slice" is already a name, and names can already be followed by square brackets.  He's just asking for a new method on a metaclass.

On Sat, Nov 12, 2016 at 1:26 AM, Steven D'Aprano <steve@pearwood.info> wrote:
On Thu, Oct 06, 2016 at 04:19:17PM -0700, Neil Girdhar wrote:
> Currently str(slice(10)) returns "slice(None, 10, None)"
> If the start and step are None, consider not emitting them.  Similarly
> slice(None) is rendered slice(None, None, None).
> When you're printing a lot of slices, it's a lot of extra noise.

I have an alternative suggestion. Wouldn't it be nice if slice objects
looked something like the usual slice syntax?

If you think the answer is No, then you'll hate my suggestion :-)

Let's keep the current repr() of slice objects as they are, using the
full function-call syntax complete with all three arguments show

    repr(slice(None, None, None)) => "slice(None, None, None)"

But let's make str() of a slice more suggestive of actual slicing, and
as a bonus, make slices easier to create too.

    str(slice(None, None, None)) => "slice[:]"

Let the slice type itself be sliceable, as an alternate constuctor:

    slice[:]            => returns slice(None)
    slice[start:]       => returns slice(start, None)
    slice[:end]         => returns slice(None, end)
    slice[start::step]  => returns slice(start, None, step)

and so forth. (This probably would require changing the type of slice to
a new metaclass.)

And then have str() return the compact slice syntax.

At worst, the compact slice syntax is one character longer than the
optimal function syntax:

    # proposed slice str()
    slice[:7]  # 9 characters

    # proposed compact str()
    slice(7)  # 8 characters

    # current str()
    slice(None, 7, None)  # 20 characters

but it will be more compact more often:

    slice[1:]  # 9 characters


    slice(1, None)  # 14 characters
    slice(None, 1, None)  # 20 characters

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