[Python-ideas] Re: slices syntactic sugar

[Ricky Teachey]

On Thu, Aug 12, 2021 at 5:16 PM Steven D'Aprano  wrote:

> On Thu, Aug 12, 2021 at 09:57:06AM -0400, Ricky Teachey wrote:
>
> > This got me thinking just now: allowing ellipses instead of None for the
> > first two arguments of the slice() constructor might be a neat idea.
>
> Like this?
>
> >>> obj = slice(..., ..., 5)
> >>> print(obj)
> slice(Ellipsis, Ellipsis, 5)
>
> The arguments to slice can be anything you like.
>
> >>> slice({'a': object()}, int, ('spam', 'eggs'))
> slice({'a': }, , ('spam',
> 'eggs'))
>
> and the interpretation is entirely up to the class:
>
> >>> class C:
> ... def __getitem__(self, obj):
> ... return obj
> ...
> >>> C()["Hello world!":[]:2.5j]
> slice('Hello world!', [], 2.5j)
>
> Aside from the use of a literal `...` for Ellipsis, which is a lot more
> recent, this behaviour goes back to Python 1.5 or older. Slices have
> always been liberal about what they accept.
>
>
> --
> Steve
>

Oh man thanks for pointing that out... and of course numpy and pandas and
many other libraries already use ellipses for various things in slices.

and of course native python sequences already have far better syntactic
sugar than ellipses:

[][3:]
[][::-1]

etc etc.

So given that, why would we ever want to use ellipses?!?!


---
Ricky.

"I've never met a Kentucky man who wasn't either thinking about going home
or actually going home." - Happy Chandler
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[Python-ideas] Re: slices syntactic sugar

[Steven D'Aprano]

On Thu, Aug 12, 2021 at 09:57:06AM -0400, Ricky Teachey wrote:

> This got me thinking just now: allowing ellipses instead of None for the
> first two arguments of the slice() constructor might be a neat idea.

Like this?

>>> obj = slice(..., ..., 5)
>>> print(obj)
slice(Ellipsis, Ellipsis, 5)

The arguments to slice can be anything you like.

>>> slice({'a': object()}, int, ('spam', 'eggs'))
slice({'a': }, , ('spam', 'eggs'))

and the interpretation is entirely up to the class:

>>> class C:
... def __getitem__(self, obj):
... return obj
... 
>>> C()["Hello world!":[]:2.5j]
slice('Hello world!', [], 2.5j)

Aside from the use of a literal `...` for Ellipsis, which is a lot more 
recent, this behaviour goes back to Python 1.5 or older. Slices have 
always been liberal about what they accept.


-- 
Steve
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[Python-ideas] Re: slices syntactic sugar

[2QdxY4RzWzUUiLuE]

On 2021-08-12 at 14:05:23 -,
eloi.riv...@aquilenet.fr wrote:

> This is how slices are used in the python standard library, indeed,
> but that does not stop me from interpreting the slices as "inclusive
> by default" in my library.  The inconsistency with the rest of the
> python standard library could be misleading, but I think maybe less
> than off-by-1 errors?

https://docs.python.org/3/library/functions.html#slice says that slice
objects represent "the set of indices specified by range(start, stop,
step)."  You are free to interpret a slice differently, but you may be
in for a fight against (or bug reports from) existing Pythonistas.

Also, given that slices explicitly represent indices, your API may be
clearer if you used ranges instead of slices, but the question of what
the end is would remain.  YMMV.

> You raise a good point however, that is: how to write a slice with
> expliciting the inclusiveness of one of the limit values?

By definition, slices derive from ranges, and by definition, ranges do
not include their ends.  A (stop, stop) pair that includes the stop
value is not a Python slice or a Python range.  If you're willing to
write your own gt class or function, then why not bt?

That said, I don't think I've ever explicitly created a slice.  And the
better iteration works, the less I use range.  I also can't recall using
BETWEEN in database queries (except maybe for dates?), but databases and
database queries are *not* my thing.  In that light, consider me -0 on
any new syntax for creating slices.
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[Python-ideas] Re: slices syntactic sugar

[Chris Angelico]

On Fri, Aug 13, 2021 at 12:00 AM Ricky Teachey  wrote:
>
> On Thu, Aug 12, 2021 at 9:02 AM Calvin Spealman  wrote:
>>
>> An alternative suggestion, which works today (... is a valid object called 
>> Ellipsis):
>>
>>Foobar.search(
>> attr1="foo",
>> attr2=[10, ...],
>> attr3=[42, ..., 50]
>> )
>
>
> This got me thinking just now: allowing ellipses instead of None for the 
> first two arguments of the slice() constructor might be a neat idea.
>
> These looks pretty nice:
>
> slice(1, ...)
> slice(1, ...,  2)
> slice(-1, ..., -1)
> slice(..., ..., 3)
>
> I wonder if-- had the ellipses existed when slice() was created eons ago/in 
> the depths of time-- whether ... would have been used rather than None for 
> these.
>
> I'm not really proposing this, just musing.
>

Ellipsis has a different meaning. None means omitted, Ellipsis means
something that most classes ignore (Numpy uses it for "all other
dimensions").

ChrisA
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[Python-ideas] Re: slices syntactic sugar

[eloi . rivard]

Definitively not as short as what I suggested, but I love it. I think this is 
one of the cleanest way I can do right now. Thanks!
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[Python-ideas] Re: slices syntactic sugar

[eloi . rivard]

This is how slices are used in the python standard library, indeed, but that 
does not stop me from interpreting the slices as "inclusive by default" in my 
library.
The inconsistency with the rest of the python standard library could be 
misleading, but I think maybe less than off-by-1 errors?

You raise a good point however, that is: how to write a slice with expliciting 
the inclusiveness of one of the limit values?
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[Python-ideas] Re: slices syntactic sugar

[Ricky Teachey]

On Thu, Aug 12, 2021 at 9:02 AM Calvin Spealman  wrote:

> An alternative suggestion, which works today (... is a valid object called
> Ellipsis):
>
>Foobar.search(
> attr1="foo",
> attr2=[10, ...],
> attr3=[42, ..., 50]
> )
>

This got me thinking just now: allowing ellipses instead of None for the
first two arguments of the slice() constructor might be a neat idea.

These looks pretty nice:

slice(1, ...)
slice(1, ...,  2)
slice(-1, ..., -1)
slice(..., ..., 3)

I wonder if-- had the ellipses existed when slice() was created eons ago/in
the depths of time-- whether ... would have been used rather than None for
these.

I'm not really proposing this, just musing.

---
Ricky.

"I've never met a Kentucky man who wasn't either thinking about going home
or actually going home." - Happy Chandler
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[Python-ideas] Re: slices syntactic sugar

[2QdxY4RzWzUUiLuE]

On 2021-08-12 at 08:56:22 -,
eloi.riv...@aquilenet.fr wrote:

> Hi. I am working on a kinda-ORM library, which usage often implies to request 
> data within specific ranges:
> 
>  Foobar.search(
> attr1="foo",
> attr2=gt(10),
> attr3=between(42, 50)
> )
> 
> The use of "gt" or "between" methods to describe those operations feels a bit 
> cumbersome (as it is long to write, and you need to import those functions in 
> a lot of files), and I though it could be more pythonic to use slices instead.
> 
>  Foobar.search(
> attr1="foo",
> attr2=slice(10),
> attr3=slice(42, 50)
> )
> 
> I suggest an alternative way to instanciate slices, that would look like this:
> 
>  Foobar.search(
> attr1="foo",
> attr2=[10:],
> attr3=[42:50]
> )
> 
> What do you think?

I think there will be a lot of off-by-1 errors.

BETWEEN is inclusive on both ends.  BETWEEN 42 AND 50 includes records
where the value is 50.

Python slices are open ended intervals.  slice(42, 50) would *not*
include the value 50.

That tells me that slices are *not* a good substitute for BETWEEN when
it comes to database queries.
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[Python-ideas] Re: slices syntactic sugar

[Calvin Spealman]

An alternative suggestion, which works today (... is a valid object called
Ellipsis):

   Foobar.search(
attr1="foo",
attr2=[10, ...],
attr3=[42, ..., 50]
)

On Thu, Aug 12, 2021 at 8:44 AM  wrote:

> Hi. I am working on a kinda-ORM library, which usage often implies to
> request data within specific ranges:
>
>  Foobar.search(
> attr1="foo",
> attr2=gt(10),
> attr3=between(42, 50)
> )
>
> The use of "gt" or "between" methods to describe those operations feels a
> bit cumbersome (as it is long to write, and you need to import those
> functions in a lot of files), and I though it could be more pythonic to use
> slices instead.
>
>  Foobar.search(
> attr1="foo",
> attr2=slice(10),
> attr3=slice(42, 50)
> )
>
> I suggest an alternative way to instanciate slices, that would look like
> this:
>
>  Foobar.search(
> attr1="foo",
> attr2=[10:],
> attr3=[42:50]
> )
>
> What do you think?
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>
>

-- 

CALVIN SPEALMAN

SENIOR QUALITY ENGINEER

calvin.speal...@redhat.com  M: +1.336.210.5107
[image: https://red.ht/sig] 
TRIED. TESTED. TRUSTED. 
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[Python-ideas] slices syntactic sugar

[eloi . rivard]

Hi. I am working on a kinda-ORM library, which usage often implies to request 
data within specific ranges:

 Foobar.search(
attr1="foo",
attr2=gt(10),
attr3=between(42, 50)
)

The use of "gt" or "between" methods to describe those operations feels a bit 
cumbersome (as it is long to write, and you need to import those functions in a 
lot of files), and I though it could be more pythonic to use slices instead.

 Foobar.search(
attr1="foo",
attr2=slice(10),
attr3=slice(42, 50)
)

I suggest an alternative way to instanciate slices, that would look like this:

 Foobar.search(
attr1="foo",
attr2=[10:],
attr3=[42:50]
)

What do you think?
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