Seek support for new slice syntax PEP.
Dave
should be available using [start:end] syntax. Following example comes
from projecteuler.net problem 166. The Numeric community would also
like this, as would the general python user. The slice notation would
require one ":" between the brackets to differentiate it from a list,
which is similar to the set notation requirement that disambiguates it
from a dictionary.
Several times now I've wanted python slice notation. Perhaps I'll
write a Python Enhancement Proposal. I stored slices of vector array
entries to add
edge = 4
indexes = []
n = edge
nn = n**2
for i in range(edge):
indexes.extend([
slice(i*n,(i+1)*n,1), # rows
slice(i,nn,n), # cols
])
row_slices = indexes[0::2]
col_slices = indexes[1::2]
slash = slice(n-1,n*(n-1)+1,n-1)
backslash = slice(0,nn,n+1)
Which could have been written in a manner completely consistent with
other python shorthand notations and for which python "cannot
possibly" use the notation for some other purpose,
edge = 4
indexes = []
n = edge
nn = n**2
for i in range(edge):
indexes.extend([
[i*n: (i+1)*n] # rows
[i: nn: n], # cols
])
row_slices = indexes[0::2]
col_slices = indexes[1::2]
slash = [n-1: n*(n-1)+1: n-1]
backslash = [0: nn: n+1]
geremy condra
Geremy Condra
Colin W.
Yes, we know that PEP 3003 applies but I see no harm in discussing
possible enhancements.
The existing slice seems a little different from what you are proposing:
An object usually containing a portion of a sequence. A slice is created
using the subscript notation, [] with colons between numbers when
several are given, such as in variable_name[1:3:5].
or:
Slice objects
Slice objects are used to represent slices when extended slice syntax is
used. This is a slice using two colons, or multiple slices or ellipses
separated by commas, e.g., a[i:j:step], a[i:j, k:l], or a[..., i:j].
They are also created by the built-in slice() function.
If your scheme flies, would it be practicable to use the same syntax
as a range generator?
range(i, j, k) => i:j:k
so range(10, 2) => :10:2
i.e. we could write for i in :10:2:
or the more common:
range(10) => :10
Colin W.
geremy condra
> enhancements.
I don't think the OP knew that the moratorium was in effect. That's why I
brought it up.
Geremy Condra
Terry Reedy
> Just as sets may now be written as {3,'hi'}, I propose that slices
> should be available using [start:end] syntax.
I believe this has been proposed and rejected on one of the py-dev,
py-ideas, or py-3k lists, but I would have to check to be sure.
Extended slices would also have to be allowed.
> The Numeric community would also like this,
Evidence? Are you one of the leaders thereof?
> as would the general python user.
A few might but most would find it useless since they never write
explicit slice objects and would have to learning something new to read
code like the below.
Many more people uses range objects (xrange in 2.x). A range object has
the same info as a slice object *plus* it is iterable. So it would be
MUCH more useful if that notation created a range object.
for i in [1:n]: ...
So I would oppose the slice proposal in favor of a range proposal.
However, his has also, I believe, been rejected, as an abbreviation too far.
> Several times now I've wanted python slice notation. Perhaps I'll
> write a Python Enhancement Proposal.
That could be useful, even if it gets rejected. Or perhaps this should
be added to 3099.
> edge = 4
> indexes = []
> n = edge
> nn = n**2
> for i in range(edge):
> indexes.extend([
> slice(i*n,(i+1)*n,1), # rows
> slice(i,nn,n), # cols
> ])
>
> row_slices = indexes[0::2]
> col_slices = indexes[1::2]
> slash = slice(n-1,n*(n-1)+1,n-1)
> backslash = slice(0,nn,n+1)
>
> Which could have been written in a manner completely consistent with
> other python shorthand notations
Python avoids getting to chicken-scratchy. There was even a proposal
(rejected, see 3099) to deprecate [1,2,3], etc, in favor of list(1,2,3),
etc.
> and for which python "cannot possibly" use the notation for some
other purpose,
But it could, see above.
> edge = 4
> indexes = []
> n = edge
> nn = n**2
> for i in range(edge):
> indexes.extend([
> [i*n: (i+1)*n] # rows
> [i: nn: n], # cols
> ])
>
> row_slices = indexes[0::2]
> col_slices = indexes[1::2]
> slash = [n-1: n*(n-1)+1: n-1]
> backslash = [0: nn: n+1]
I find this currently to be less readable.
Terry Jan Reedy
Terry Reedy
> http://www.python.org/dev/peps/pep-3003/
The moratorium does not stop proposals for things to be added after the
moratorium ends. But it does show that Guido and the devs are reluctant
to make *any* change to the core syntax of 3.x without really good
reason. Absent that, I would not mind if the syntax remains frozen for
the rest of 3.x. A minor abbreviation that makes the language look more
like Perl will not cut it.
Terry Jan Reedy
Lie Ryan
> Just as sets may now be written as {3,'hi'}, I propose that slices
> should be available using [start:end] syntax. Following example comes
> from projecteuler.net problem 166. The Numeric community would also
> like this, as would the general python user. The slice notation would
> require one ":" between the brackets to differentiate it from a list,
> which is similar to the set notation requirement that disambiguates it
> from a dictionary.
I would prefer [a: b, ...] syntax to become an ordered dictionary
literal (if it would ever gain traction).
Steven D'Aprano
> Yes, we know that PEP 3003 applies but I see no harm in discussing
> possible enhancements.
You bored? Looking for something to do?
I've lost all enthusiasm for discussing language enhancements, regardless
of whether I'm for or against the change, knowing that there's no way it
could be added to the language, and when the Python moratorium ends the
discussion will just happen all over again.
--
Steven
Carl Banks
-1
Explicit creation of slice objects is an uncommon need and there is no
reason to support it with its own syntax.
I'd agree with Terry Reedy that range/xrange is far more commonly used
than slice objects, and if a floating slice syntax were ever added to
Python it ought to be used for range.
If you need to use a lot of slice objects you can lower your code
footprint by defining a helper class like this (adapt as needed):
class SliceCreator(object):
def __getitem__(self,loc):
if not isinstance(loc,slice):
raise TypeError
return loc
slc = SliceCreator()
slash = slc[n-1: n*(n-1)+1: n-1]
It might have been a reasonable idea for slice (and, perhaps, range)
to use slice notation rather than a function call, on the thinking
that the notational convenience outweighs the fact that you're not
actually getting an item, but it's too late for that.
Carl Banks
Nobody
> Just as sets may now be written as {3,'hi'}, I propose that slices
> should be available using [start:end] syntax. Following example comes
> from projecteuler.net problem 166. The Numeric community would also
> like this, as would the general python user. The slice notation would
> require one ":" between the brackets to differentiate it from a list,
> which is similar to the set notation requirement that disambiguates it
> from a dictionary.
>
> Several times now I've wanted python slice notation. Perhaps I'll
> write a Python Enhancement Proposal.
Would it suffice to add the equivalent of numpy.s_ as a builtin?
> from numpy import s_
> s_[1:2:3]
slice(1, 2, 3)
> s_[1:2:3, ..., 4:5]
(slice(1, 2, 3), Ellipsis, slice(4, 5, None))
Or would it be possible to define "slice" itself so that it implements
__getitem__ and __getslice__?
Rhodri James
wrote:
> If your scheme flies, would it be practicable to use the same syntax
> as a range generator?
>
> range(i, j, k) => i:j:k
>
> so range(10, 2) => :10:2
>
> i.e. we could write for i in :10:2:
>
> or the more common:
> range(10) => :10
Ugh. Magic characters. Let's not.
--
Rhodri James *-* Wildebeest Herder to the Masses
Anh Hai Trinh
> > s_[1:2:3]
> slice(1, 2, 3)
> > s_[1:2:3, ..., 4:5]
> (slice(1, 2, 3), Ellipsis, slice(4, 5, None))
>
> Or would it be possible to define "slice" itself so that it implements
> __getitem__ and __getslice__?
Indeed!
Python 2.6.4 (r264:75706, Oct 27 2009, 06:25:13)
[GCC 4.4.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class slice(object):
... @staticmethod
... def __getitem__(sliceobj):
... return sliceobj
>>> slice = slice()
>>> slice[:]
slice(None, None, None)
>>> slice[1::-1]
slice(1, None, -1)
>>> range(10).__getitem__(slice[::2])
[0, 2, 4, 6, 8]
----aht
Bearophile
> I've lost all enthusiasm for discussing language enhancements
That's probably the main downside of the moratorium. Humans need to
play some to keep their will to work and improve things.
Bye,
bearophile
r0g
I agree, string slicing syntax is already a little oblique, it certainly
doesn't need complicating.
Anyway...
Simple is better than complex.
Readability counts.
If the implementation is hard to explain, it's a bad idea.
Roger.
Gregory Ewing
> So it would be
> MUCH more useful if that notation created a range object.
>
> for i in [1:n]: ...
>
> So I would oppose the slice proposal in favor of a range proposal.
Another possibility would be to unify range and slice
objects so that they're actually the same thing. Then
the same notation could be used for both purposes.
--
Greg
Colin W.
Terry Reedy suggested:- for i in [1:n]: ...
Are the brackets really needed?
Colin W.
Nobody
> Many more people uses range objects (xrange in 2.x). A range object has
> the same info as a slice object *plus* it is iterable.
This isn't quite true, as a range cannot have a stop value of None, i.e.
you can't represent [n:] or [:] etc as a range. Similarly for using
negative stop values for indices relative to the end of the sequence being
sliced.
Also, aside from the semantics of slice objects themselves, slice notation
isn't limited to a single slice object; it can also return a tuple of
slices and values, e.g.:
> numpy.s_[1::2,...,3,4:5:6]
(slice(1, None, 2), Ellipsis, 3, slice(4, 5, 6))
For a single slice, enumerating over a slice with an unspecified stop
value would be equivalent to itertools.count(). Negative stop values won't
work.
For a multi-dimensional slice, with everything specified, you would
probably want to iterate over the cartesian product (i.e. N nested loops
for an N-dimensional slice). But this won't work if anything other than
the outermost loop has an unspecified stop value, or if you use an
ellipsis within a slice.
Oh, and being able to slice a slice could be quite useful, i.e.:
[10:90:10][2::2] == [30:90:20]
cf:
> numpy.arange(100)[10:90:10][2::2]
array([30, 50, 70])
> numpy.arange(100)[30:90:20]
array([30, 50, 70])
Colin W.
Section 5.3.3 of the Python doc for 2.6.4 has
The lower and upper bound expressions, if present, must evaluate to
plain integers; defaults are zero and the sys.maxint, respectively. If
either bound is negative, the sequence’s length is added to it. The
slicing now selects all items with index k such that i <= k < j where i
and j are the specified lower and upper bounds. This may be an empty
sequence. It is not an error if i or j lie outside the range of valid
indexes (such items don’t exist so they aren’t selected).
Colin W.
Colin W.
Nobody
> You don't say, but seem to imply that the slice components include None.
That's how missing components are implemented at the language level:
> class foo:
= def __getitem__(self, s):
= return s
=
> x = foo()
> x[::]
slice(None, None, None)
> x[1::2]
slice(1, None, 2)
The defaults of zero, sys.maxint and one apply to built-in types, but
nothing forces user-defined types to behave this way.
Or maybe I misunderstood your point.
Colin W.
You are right:
*** Python 2.6.4 (r264:75708, Oct 26 2009, 08:23:19) [MSC v.1500 32 bit
(Intel)] on win32. ***
>>> a= range(10)
>>> a[2:8:2]
[2, 4, 6]
>>> a[2::2]
[2, 4, 6, 8]
>>> a[2:None:2]
[2, 4, 6, 8]
>>>
I had expected the last to be rejected, but it fits with the overall
philosophy.
Colin W
Dave Angel
> <div class="moz-text-flowed" style="font-family: -moz-fixed">On
>
None is perfectly valid as a parameter to a slice. To quote the 2.6.4
docs, in section 6.6:
The slice of /s/ from /i/ to /j/ with step /k/ is defined as the
sequence of items with index x = i + n*k such that 0 <= n < (j-i)/k. In
other words, the indices are i, i+k, i+2*k, i+3*k and so on, stopping
when /j/ is reached (but never including /j/). If /i/ or /j/ is greater
than len(s), use len(s). If /i/ or /j/ are omitted or None, they become
�end� values (which end depends on the sign of /k/). Note, /k/ cannot be
zero. If /k/ is None, it is treated like 1.
DaveA