slicing lists

[Yves Dorfsman]

Is there a way to do:
x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
x[0,2:6]

That would return:
[0, 3, 4, 5, 6]

I am surprised this notation is not supported, it seems intuitive.
A concrete example of the sort of thing I want to do:

p = file('/etc/passwd').readli nes()
q = [ e.strip().split (':')[0,2:] for e in p ]

(getting rid of the password / x field)


Thanks.

Yves.

http://www.SollerS.ca

[Ivan Illarionov]

Re: slicing lists

On Wed, 07 May 2008 23:29:27 +0000, Yves Dorfsman wrote:

Is there a way to do:
x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
x[0,2:6]
>
That would return:
[0, 3, 4, 5, 6]

IMHO this notation is confusing.

What's wrong with:
[0]+x[2:6]

I am surprised this notation is not supported, it seems intuitive. A
concrete example of the sort of thing I want to do:
>
p = file('/etc/passwd').readli nes()
q = [ e.strip().split (':')[0,2:] for e in p ]
>
(getting rid of the password / x field)

This works and is clearer:
[[0] + e.strip().split (':')[2:] for e in open('/etc/passwd')]

--
Ivan

[Ivan Illarionov]

Re: slicing lists

On Wed, 07 May 2008 23:46:33 +0000, Ivan Illarionov wrote:

On Wed, 07 May 2008 23:29:27 +0000, Yves Dorfsman wrote:
>

>Is there a way to do:
>x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>x[0,2:6]
>>
>That would return:
>[0, 3, 4, 5, 6]

>
IMHO this notation is confusing.
>
What's wrong with:
[0]+x[2:6]
>

>I am surprised this notation is not supported, it seems intuitive. A
>concrete example of the sort of thing I want to do:
>>
>p = file('/etc/passwd').readli nes()
>q = [ e.strip().split (':')[0,2:] for e in p ]
>>
>(getting rid of the password / x field)

>
This works and is clearer:
[[0] + e.strip().split (':')[2:] for e in open('/etc/passwd')]

or maybe you wanted to do this:

>>[e.split(':')[0] for e in open('/etc/passwd')]

['root', 'daemon', 'bin', 'sys', 'sync', ...]

What are you trying to get from /etc/passwd?

--
Ivan

[castironpi@gmail.com]

Re: slicing lists

On May 7, 6:58 pm, Ivan Illarionov <ivan.illario.. .@gmail.comwrot e:

On Wed, 07 May 2008 23:46:33 +0000, Ivan Illarionov wrote:

On Wed, 07 May 2008 23:29:27 +0000, Yves Dorfsman wrote:

>

Is there a way to do:
x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
x[0,2:6]

>

That would return:
[0, 3, 4, 5, 6]

>

IMHO this notation is confusing.

>

What's wrong with:
[0]+x[2:6]

>

I am surprised this notation is not supported, it seems intuitive. A
concrete example of the sort of thing I want to do:

>

p = file('/etc/passwd').readli nes()
q = [ e.strip().split (':')[0,2:] for e in p ]

>

(getting rid of the password / x field)

>

This works and is clearer:
[[0] + e.strip().split (':')[2:] for e in open('/etc/passwd')]

>
or maybe you wanted to do this:
>

>[e.split(':')[0] for e in open('/etc/passwd')]

>
['root', 'daemon', 'bin', 'sys', 'sync', ...]
>
What are you trying to get from /etc/passwd?
>
--
Ivan- Hide quoted text -
>
- Show quoted text -

The t. Oughtn't there be a run on Guido's name more often than is?

You -could- write:

x[ slice(0)+slice( 2,6) ]

where slice would be an 'autochaining' type under a syntax. Fine for
slices, not for logic.

x[ b1+And+b2+Or+b3 ]

would also register as b1 and b2 or b3, which really quickly rises on
the wheel again, er, um, crawls up the spout.

Would you be happy with x[ '0,2:6' ], necessarily in quotes? With two
punctuation marks you couldn't get much farther in money than Python
does today.

x[ '0 2:6' ]
-x.__getitem__( x, '0 2:6' )
-'0 2:6'.split( )
-x.__getslice__( x, slice( 0 ) )
... + x.__getslice__( x, slice( 2, 6 ) )

However, it's not clear it's trivial to overwrite a built-in type's
__getitem__!

[Miles]

Re: slicing lists

On Wed, May 7, 2008 at 7:46 PM, Ivan Illarionov
<ivan.illariono v@gmail.comwrot e:

On Wed, 07 May 2008 23:29:27 +0000, Yves Dorfsman wrote:
>

Is there a way to do:
x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
x[0,2:6]
>
That would return:
[0, 3, 4, 5, 6]

>
IMHO this notation is confusing.
>
What's wrong with:
[0]+x[2:6]

I think Yves meant to return [1, 3, 4, 5, 6], as in Perl's list slicing:

my @x = (1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
return @x[0, 2..6]; // returns (1, 3, 4, 5, 6)

This isn't incredibly efficient, but it does what you want (I think):

from itertools import chain

class multisliceable( list):
def __getitem__(sel f, slices):
if isinstance(slic es, (slice, int, long)):
return list.__getitem_ _(self, slices)
else:
return list(chain(*[list.__getitem_ _(self, s) if isinstance(s, slice)
else[list.__getitem_ _(self, s)] for s in slices]))

p = open('/etc/passwd')
q = [multisliceable( e.strip().split (':'))[0,2:] for e in p]

-Miles

[Ivan Illarionov]

Re: slicing lists

On Wed, 07 May 2008 21:13:27 -0400, Miles wrote:

On Wed, May 7, 2008 at 7:46 PM, Ivan Illarionov
<ivan.illariono v@gmail.comwrot e:

On Wed, 07 May 2008 23:29:27 +0000, Yves Dorfsman wrote:
>

Is there a way to do:
x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
x[0,2:6]
>
That would return:
[0, 3, 4, 5, 6]

>
IMHO this notation is confusing.
>
What's wrong with:
[0]+x[2:6]

>
I think Yves meant to return [1, 3, 4, 5, 6], as in Perl's list
slicing:
>
my @x = (1, 2, 3, 4, 5, 6, 7, 8, 9, 10); return @x[0, 2..6]; // returns
(1, 3, 4, 5, 6)

So it should be x[0] + x[2:6] or x[0].extend(x[2:6])

[Ivan Illarionov]

Re: slicing lists

On Thu, 08 May 2008 01:15:43 +0000, Ivan Illarionov wrote:

On Wed, 07 May 2008 21:13:27 -0400, Miles wrote:
>

>On Wed, May 7, 2008 at 7:46 PM, Ivan Illarionov
> <ivan.illariono v@gmail.comwrot e:

> On Wed, 07 May 2008 23:29:27 +0000, Yves Dorfsman wrote:
> >
> Is there a way to do:
> x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
> x[0,2:6]
> >
> That would return:
> [0, 3, 4, 5, 6]
> >
> IMHO this notation is confusing.
> >
> What's wrong with:
> [0]+x[2:6]

>>
> I think Yves meant to return [1, 3, 4, 5, 6], as in Perl's list
> slicing:
>>
> my @x = (1, 2, 3, 4, 5, 6, 7, 8, 9, 10); return @x[0, 2..6]; //
> returns (1, 3, 4, 5, 6)

>
So it should be x[0] + x[2:6] or x[0].extend(x[2:6])

Oh, I meant
[x[0]] + x[2:6]
or
y = [x[0]]
y.extend(x[2:6])

Sorry

[Yves Dorfsman]

Re: slicing lists

Miles wrote:

On Wed, May 7, 2008 at 7:46 PM, Ivan Illarionov

Is there a way to do:
x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
x[0,2:6]
>
That would return:
[0, 3, 4, 5, 6]

Arg... Yes, this is a typo, I meant:
[1, 3, 4, 5, 6]

I think Yves meant to return [1, 3, 4, 5, 6], as in Perl's list slicing:
>
my @x = (1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
return @x[0, 2..6]; // returns (1, 3, 4, 5, 6)

Yes, exactly.

>
This isn't incredibly efficient, but it does what you want (I think):
>
from itertools import chain
>
class multisliceable( list):
def __getitem__(sel f, slices):
if isinstance(slic es, (slice, int, long)):
return list.__getitem_ _(self, slices)
else:
return list(chain(*[list.__getitem_ _(self, s) if isinstance(s, slice)
else[list.__getitem_ _(self, s)] for s in slices]))
>
p = open('/etc/passwd')
q = [multisliceable( e.strip().split (':'))[0,2:] for e in p]

So would it be a worthy addition to python, to add it right in the core of
the language, and hopefully in an efficient manner ?

That would certainly help some type of list comprehensions, making them more
readable, and hopefully more readable (running split once instead of twice,
or how many number of time you need it). The passwd example is just one
example I ran into, but I can see running in this problem a lot with more
complex cases. Right now I solve the passwd pb with:

p = file('/etc/passwd').readli nes()
r = [ e.strip().split (':') for e in p ]
s = [ e[0:1] + e[2:] for e in r ]


Or:

p = file('/etc/passwd').readli nes()
s = [ e.strip().split (':')[0:1] + e.strip().split (':')[2:] for e in p ]

In the first case we're looping twice (two list comprehension), in the
second case we're running the split twice on every element of p.


Yves.

http://www.SollerS.ca

[George Sakkis]

Re: slicing lists

On May 7, 11:34 pm, Yves Dorfsman <y...@zioup.com wrote:

So would it be a worthy addition to python, to add it right in the core of
the language, and hopefully in an efficient manner ?

Given that it's a straightforward generalization of the existing
slicing syntax, it sure does make sense from a theoretical standpoint.
Whether it's a worthy addition is debatable though; I don't think
there are many common use cases to convince the core developers to
work on it. OTOH if you (or someone else) comes up with a working
patch, it might improve its chances of getting accepted.

George

[Boris Borcic]

Re: slicing lists

Yves Dorfsman wrote:

>
So would it be a worthy addition to python, to add it right in the core
of the language, and hopefully in an efficient manner ?
>

Note that the s[0,2:6] syntax is currently allowed because of the distinct
semantics that the Numeric module and its successors numarray and numpy had for it.

[MRAB]

Re: slicing lists

On May 8, 4:34 am, Yves Dorfsman <y...@zioup.com wrote:

Miles wrote:

On Wed, May 7, 2008 at 7:46 PM, Ivan Illarionov

Is there a way to do:
x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
x[0,2:6]

>

That would return:
[0, 3, 4, 5, 6]

>
Arg... Yes, this is a typo, I meant:
[1, 3, 4, 5, 6]
>

I think Yves meant to return [1, 3, 4, 5, 6], as in Perl's list slicing:

>

my @x = (1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
return @x[0, 2..6]; // returns (1, 3, 4, 5, 6)

>
Yes, exactly.
>
>
>

This isn't incredibly efficient, but it does what you want (I think):

>

from itertools import chain

>

class multisliceable( list):
def __getitem__(sel f, slices):
if isinstance(slic es, (slice, int, long)):
return list.__getitem_ _(self, slices)
else:
return list(chain(*[list.__getitem_ _(self, s) if isinstance(s, slice)
else[list.__getitem_ _(self, s)] for s in slices]))

>

p = open('/etc/passwd')
q = [multisliceable( e.strip().split (':'))[0,2:] for e in p]

>
So would it be a worthy addition to python, to add it right in the core of
the language, and hopefully in an efficient manner ?
>
That would certainly help some type of list comprehensions, making them more
readable, and hopefully more readable (running split once instead of twice,
or how many number of time you need it). The passwd example is just one
example I ran into, but I can see running in this problem a lot with more
complex cases. Right now I solve the passwd pb with:
>
p = file('/etc/passwd').readli nes()
r = [ e.strip().split (':') for e in p ]
s = [ e[0:1] + e[2:] for e in r ]
>
Or:
>
p = file('/etc/passwd').readli nes()
s = [ e.strip().split (':')[0:1] + e.strip().split (':')[2:] for e in p ]
>
In the first case we're looping twice (two list comprehension), in the
second case we're running the split twice on every element of p.
>

You should've read the thread entitled "Why don't generators execute
until first yield?"! :-) Michael Torrie gave the URL
http://www.dabeaz.com/generators/Generators.pdf. Your example can be
rewritten as follows:

p = file('/etc/passwd') # No need for readlines() because file's
iterator yields the lines.
r = ( e.strip().split (':') for e in p ) # A generator expression
instead of a list comprehension.
s = [ e[0:1] + e[2:] for e in r ]

[Yves Dorfsman]

Re: slicing lists

MRAB wrote:

You should've read the thread entitled "Why don't generators execute
until first yield?"! :-) Michael Torrie gave the URL
http://www.dabeaz.com/generators/Generators.pdf. Your example can be
rewritten as follows:
>
p = file('/etc/passwd') # No need for readlines() because file's
iterator yields the lines.
r = ( e.strip().split (':') for e in p ) # A generator expression
instead of a list comprehension.
s = [ e[0:1] + e[2:] for e in r ]

Thanks, this is very elegant indeed, but because there are two list
comprehension expressions (the one defining r and the one defining s), it
means that the we're looping twice over the same list instead of once with
the e[0,2:] hypotetical notation ; or am I missing something ?

Yves.

http://www.SollerS.ca

[Ian Kelly]

Re: slicing lists

On Wed, May 7, 2008 at 5:29 PM, Yves Dorfsman <yves@zioup.com wrote:

Is there a way to do:
x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
x[0,2:6]
>
That would return:
[0, 3, 4, 5, 6]
>
I am surprised this notation is not supported, it seems intuitive.
A concrete example of the sort of thing I want to do:
>
p = file('/etc/passwd').readli nes()
q = [ e.strip().split (':')[0,2:] for e in p ]
>
(getting rid of the password / x field)

Have a look at the itemgetter function from the operator module.

g = itemgetter( 0, *range(2, 6) )
p = file("/etc/passwd").readli nes()
q = [ g( e.strip().split (':') ) for e in p ]

[castironpi@gmail.com]

Re: slicing lists

On May 9, 1:23 am, "Ian Kelly" <ian.g.ke...@gm ail.comwrote:

On Wed, May 7, 2008 at 5:29 PM, Yves Dorfsman <y...@zioup.com wrote:

Is there a way to do:
x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
x[0,2:6]

>

That would return:
[0, 3, 4, 5, 6]

>

I am surprised this notation is not supported, it seems intuitive.
A concrete example of the sort of thing I want to do:

>

p = file('/etc/passwd').readli nes()
q = [ e.strip().split (':')[0,2:] for e in p ]

>

(getting rid of the password / x field)

>
Have a look at the itemgetter function from the operator module.
>
g = itemgetter( 0, *range(2, 6) )
p = file("/etc/passwd").readli nes()
q = [ g( e.strip().split (':') ) for e in p ]

The only thing is, is there is another natural meaning to [a,b:c].

Counting grids on the diagonals, the rational set is well defined:

0: 0, 0
1: 1, 0
2: 0, 1
3: 2, 0
4: 1, 1
5: 0, 2
6: 3, 0
7: 2, 1
...

Thencefore ( 2, 0 ) : ( 3, 0 ) is well defined. Thencefore,

a,b:c,d

is not; x[a,b:c,d]= x[a]+ x[b:c]+ x[d].

[Yves Dorfsman]

Re: slicing lists

castironpi@gmai l.com wrote:

The only thing is, is there is another natural meaning to [a,b:c].
>
Counting grids on the diagonals, the rational set is well defined:
>
0: 0, 0
1: 1, 0
2: 0, 1
3: 2, 0
4: 1, 1
5: 0, 2
6: 3, 0
7: 2, 1
...
>
Thencefore ( 2, 0 ) : ( 3, 0 ) is well defined. Thencefore,
>
a,b:c,d
>
is not; x[a,b:c,d]= x[a]+ x[b:c]+ x[d].

I'm not sure what you mean here. Could you give me a simple piece of code to
show an example ?


Yves.

http://www.SollerS.ca