On Dec 14, 10:03 am, Dave <b49p23t...@stny.rr.com> wrote:
> 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. 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]
-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
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