Possible improvement to slice opperations.

[Ron Adam]

After considering several alternatives and trying out a few ideas with a
modified list object Bengt Richter posted, (Thank You), I think I've
found a way to make slice operation (especially far end indexing)
symmetrical and more consistent.

So to find out if this is indeed a possibility, it would be nice to get
a few opinions at this point. So blast away... or hopefully tell me
what you like about it instead. ;-)

(Any suggestions or contributions to make it better would be appreciated.)

Cheers,
Ron Adam



"""
IMPROVED SLICING
================

Slicing is one of the best features of Python in my opinion, but
when you try to use negative index's and or negative step increments
it can be tricky and lead to unexpected results.

This topic has come up fairly often on comp.lang.python, and often
times, the responses include:

* Beginners should avoid negative extended slices.

* Slices with negative step increments are for advanced
python programmers.

* It's not broke if you look at it in a different way.

* You should do it a different way.

All of these and various responses similar to them are unsatisfactory in
my opinion and it's a good indicator it's not as good as it could be.
Indexing and slice operations are vary common in Python and I don't
think we should be discouraging people from learning and using them.


COMPATIBILITY
-------------
Because the following suggested changes will break current code,
it probably can not be implemented prior to Python 3000.

+ Direct indexing with both positive and negative values
returns the same items as they do now.

+ Extended slices with all positive and or empty default
values remain unchanged.

- Extended slices with negative values return values that
have less items than currently.

- Slices with negative step values return entirely different
results.


REVERSE ORDER STEPPING
----------------------
When negative steps are used, a slice operation
does the following. (or the equivalent)

1. reverse the list
2. cut the reversed sequence using start and stop
3. iterate forward using the absolute value of step.

* This order results in an inverse selection and I believe should be
considered a bug.

Changing the order in the following way results in a much
more predictable pattern which is both easier to understand and use.

1. cut sequence using start and stop.
2 reverse the order of the results.
3. iterate forward using the absolute value of step.


CURRENT INDEXING
----------------

Given a range [a,b,c]:

Positive indexing

| a | b | c |
+---+---+---+
0 1 2 3

Current negative indexing.

| a | b | c |
+---+---+---+
-3 -2 -1 -0


When a single index is used the item to the
right of the index for both positive and
negative index's is returned.

With slices, the items between start, and
stop index's are returned.

Accessing a range at the end of a list numerically
becomes inconvenient when negative index's are used
as the '-0'th position can not be specified numerically
with negative values.


ONES BASED NEGATIVE INDEXING
----------------------------
Making negative index's Ones based, and selecting
individual item to the left of negative index's would enable
addressing the end of the list numerically.

Ones based negative index's.

| a | b | c |
+---+---+---+
-4 -3 -2 -1

Then:

a[-1] -> c # item left of index, same result as now.

a[-3:-2] -> b # item between index's

a[-1:-1] = [d] # insert item at the end.



USE OF '~' IN PLACE OF NEGATIVE INDEX'S
---------------------------------------

The '~' is the binary not symbol which when used
with integers returns the two's compliment. This
works nice with indexing from the end of a list
because convieniently ~0 == -1.

This creates a numerical symmetry between positive
indexing and '~' "End of list" indexing.

a[0] -> first item in the list.
a[~0] -> last item in the list.

a[0:~0] -> whole list.

a[1:~1] -> center, one position from both ends.

* Note: using '~' works as described here in place of single negative
index's in current versions of Python. It does not work as described
here for extended slices.

"""



# TEST LIST CLASS.
"""
A list class to Test '~' end of list indexing.

* This class modifies the slice before returning
a value. The final implementation may do this by
modifying slice objects directly or the underlying
C code of sequences.

"""

class nxlist(object):

def __init__(self, value):
self.value = value

def normslc(self, slc):
start,stop,step = slc.start, slc.stop, slc.step
if type(start) == int and start<0:
start = len(self.value)+start+1
if type(stop) == int and stop<0:
stop = len(self.value)+stop+1
return slice(start,stop,step)

def __getitem__(self, i):
tp = i.__class__
if tp == int:
if i>=0:
return self.value
else:
return self.value[ len(self.value)+i ]
if tp == slice:
slc = self.normslc(i)
value = self.value[slc.start:slc.stop]
if type(i.step) == int and i.step<0:
value.reverse()
slc = slice(None,None,-i.step)
else:
slc = slice(None,None,i.step)
return value[slc]

#def __setitem__(self, i, v):
#Not emplimented yet.

def __repr__(self): return 'nxlist(%r)'%self.value


a = nxlist(range(10))
print a

testdata = [
('a[0]'),
('a[1]'),
('a[~0]'),
('a[~1]'),
('a[:]'),
('a[:~1]'),
('a[~1:]'),
('a[::]'),
('a[0:~0]'),
('a[1:~1]'),
('a[~1:1]'),
('a[::-2]'),
('a[:3]'),
('a[3:]'),
('a[~3:]'),
('a[:~3]'),
('a[:3:-1]'),
('a[3::-1]'),
('a[~3::-1]'),
('a[:~3:-1]'),
('a[:3:-2]'),
('a[3::-2]'),
('a[~3::-2]'),
('a[:~3:-2]'),
]

for n, s in enumerate(testdata):
print '%r. %s = %r' % (n,s,eval(s))


"""
nxlist([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
0. a[0] = 0
1. a[1] = 1
2. a[~0] = 9
3. a[~1] = 8
4. a[:] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
5. a[:~1] = [0, 1, 2, 3, 4, 5, 6, 7, 8]
6. a[~1:] = [9]
7. a[::] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
8. a[0:~0] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
9. a[1:~1] = [1, 2, 3, 4, 5, 6, 7, 8]
10. a[~1:1] = []
11. a[::-2] = [9, 7, 5, 3, 1]
12. a[:3] = [0, 1, 2]
13. a[3:] = [3, 4, 5, 6, 7, 8, 9]
14. a[~3:] = [7, 8, 9]
15. a[:~3] = [0, 1, 2, 3, 4, 5, 6]
16. a[:3:-1] = [2, 1, 0]
17. a[3::-1] = [9, 8, 7, 6, 5, 4, 3]
18. a[~3::-1] = [9, 8, 7]
19. a[:~3:-1] = [6, 5, 4, 3, 2, 1, 0]
20. a[:3:-2] = [2, 0]
21. a[3::-2] = [9, 7, 5, 3]
22. a[~3::-2] = [9, 7]
23. a[:~3:-2] = [6, 4, 2, 0]
"""


r = range(10)
a = nxlist(r)

print r[~0],a[~0] # ok
print r[~3:],a[~3:] # one off
print r[~3::-1],a[~3::-1] # other side
print r[~3::-2],a[~3::-2] # other side


"""
Comparisons of negative indexing and '~'
indexing with same values.

current, proposed

9 9
[6, 7, 8, 9] [7, 8, 9]
[6, 5, 4, 3, 2, 1, 0] [9, 8, 7]
[6, 4, 2, 0] [9, 7]
"""

 

[Terry Reedy]

Slicing is one of the best features of Python in my opinion, but
when you try to use negative index's and or negative step increments
it can be tricky and lead to unexpected results.

This topic has come up fairly often on comp.lang.python, and often times,
the responses include:
* Beginners should avoid negative extended slices.
* Slices with negative step increments are for advanced
python programmers.
* It's not broke if you look at it in a different way.
* You should do it a different way.

You omitted the slice-specific response I gave before and repeat here with
more detail by quoting from What's New in Python 2.3.
http://www.python.org/doc/2.3/whatsnew/section-slices.html
"
15 Extended Slices
Ever since Python 1.4, the slicing syntax has supported an optional third
``step'' or ``stride'' argument. For example, these are all legal Python
syntax: L[1:10:2], L[:-1:1], L[::-1]. This was added to Python at the
request of the developers of Numerical Python, which uses the third
argument extensively. However, Python's built-in list, tuple, and string
sequence types have never supported this feature, raising a TypeError if
you tried it.
"
Again, extended slices were probably designed by and certainly designed for
Numerical Python and for 7 years were used at least mainly by Numerical
Python. They were not designed for other users like you. The responses
you summarized and distain pretty much all derive from this history.

So, I am pretty sure that changes to the core would have to be upwards
compatible with current usage. On the other hand, your nxlist subclass of
list seems to work pretty well now.The 2.2+ ability to do this sort of
thing is what type subclassing was made for.

Or one could just write an extended slice function similar to your .normslc
method. I might even use such a thing one day.

Terry J. Reedy

 

[Patrick Maupin]

After considering several alternatives and trying out a few ideas with a

modified list object Bengt Richter posted, (Thank You), I think I've
found a way to make slice operation (especially far end indexing)
symmetrical and more consistent.

I don't know that it makes it more consistent. I could be persuaded,
but it would have to be by real-life examples with calculated slice
indices and stride. I do this thing all the time, and find the current
rules simple and very consistent. Occasionally, I might wish that
things were a little different, but there is always a workaround. I
would have to see some real code examples, of sufficient scope to see
that there are fewer workarounds with this proposal than with the
current implementation.

FWIW, there is a reasonable workaround for the case where the indices
might be negative and you would like zero or greater to mean 'end of
list'. If "x" is the index variable, you can use the expression (x<0
and x or None) for the index value in the slice. If you find yourself
doing this often, you can write a little function for it -- def
EndIndex(x): return x<0 and x or None.

But in real code, I fear you might need a similar helper function for
similar issues with your change. I just don't know what those are
without more thought.

Regards,
Pat

 

[Ron Adam]

Slicing is one of the best features of Python in my opinion, but
when you try to use negative index's and or negative step increments
it can be tricky and lead to unexpected results.

This topic has come up fairly often on comp.lang.python, and often times,
the responses include:
* Beginners should avoid negative extended slices.
* Slices with negative step increments are for advanced
python programmers.
* It's not broke if you look at it in a different way.
* You should do it a different way.

You omitted the slice-specific response I gave before and repeat here with
more detail by quoting from What's New in Python 2.3.
http://www.python.org/doc/2.3/whatsnew/section-slices.html
"
15 Extended Slices
Ever since Python 1.4, the slicing syntax has supported an optional third
``step'' or ``stride'' argument. For example, these are all legal Python
syntax: L[1:10:2], L[:-1:1], L[::-1]. This was added to Python at the
request of the developers of Numerical Python, which uses the third
argument extensively. However, Python's built-in list, tuple, and string
sequence types have never supported this feature, raising a TypeError if
you tried it.
"

I didn't omit anything, I previously gave examples of using various
forms of slices with the intent of showing how some of those forms are
not symmetrical and difficult to understand. And then you explained
more precisely how those slices were indexed.

It would be useful to do a side by side comparison of those cases and
similar ones to this new method. Then maybe the benefits of making the
changes I'm suggesting will be clearer.

I looked at the documents in Numerical Python. From what I could tell,
it uses the form L[::-1] quite a bit, which will still work *exactly* as
it does now. There were no examples of partial negative step
operations, so I couldn't determine if/how they are used. I'll install
it and experiment with it and see if/how it works with Numerical Python.

After Python 3000 is released Numerical Python will need to be updated
even without these changes. But it is a good idea to consult with the
Numerical Python developers on this.

Again, extended slices were probably designed by and certainly designed for
Numerical Python and for 7 years were used at least mainly by Numerical
Python. They were not designed for other users like you. The responses
you summarized and distain pretty much all derive from this history.

Other users like me? And what kind of user am I?

Is it really a good idea to maintain that some things shouldn't be done
by some, but is ok for others?

I think you mean to say that extended slices with negative strides,
should only (or primarily) be used with Numerical Python. I however
disagree with that view. I think they have a broader range of uses.

So, I am pretty sure that changes to the core would have to be upwards
compatible with current usage. ...

This is why I said these changes probably couldn't be made before Python
3000. It is upward (backward?) compatible for the majority of use
cases, all direct indexing operations, and all positive extended slices.
In Python's library, only negative extended slices will need to be
adjusted. As far as I could tell there are no partial negative strides
cases that will need to be adjusted. I wish there was a few, because
then I could show how this would improve those cases.

Now even though this suggestion may be sound, that alone isn't reason
enough to include it in Python 3000. It would need clear use case
benefits and support by users as well as support from Guido.

At this point, I'm only trying to clarify the issues and develop working
examples.

... On the other hand, your nxlist subclass of list seems to work
pretty well now. The 2.2+ ability to do this sort of thing is what
type subclassing was made for.

Yes, it does work well, (so far) Which is the motivation for me
posting it. Lets make it better.

Or one could just write an extended slice function similar to your .normslc
method. I might even use such a thing one day.

Unfortunately one can not sub-class slice objects directly. And the ':'
in a slice index isn't a true operator with it's own methods. So
changing existing slice behavior will require changing the core. But
for now I will write a function and a complete list sub-class and post
it here.

Cheers,
Ron

 

[Ron Adam]

I don't know that it makes it more consistent. I could be persuaded,
but it would have to be by real-life examples with calculated slice
indices and stride. I do this thing all the time, and find the current
rules simple and very consistent. Occasionally, I might wish that
things were a little different, but there is always a workaround. I
would have to see some real code examples, of sufficient scope to see
that there are fewer workarounds with this proposal than with the
current implementation.

I'll post a few examples once I'm sure the list object works correctly
and then you can play around with it and try it out as well.

FWIW, there is a reasonable workaround for the case where the indices
might be negative and you would like zero or greater to mean 'end of
list'. If "x" is the index variable, you can use the expression (x<0
and x or None) for the index value in the slice. If you find yourself
doing this often, you can write a little function for it -- def
EndIndex(x): return x<0 and x or None.

Thanks, this is a good example.

Yes it does remove the need for a work around in those cases.

You still need to check for cases where you may cross the -1,0 boundary
while incrementing or decrementing an index. But that is an expected
condition and easily handled with an 'if' or 'for' loop.

But in real code, I fear you might need a similar helper function for
similar issues with your change. I just don't know what those are
without more thought.

Regards,
Pat

Thanks for the feedback, it was helpful.

Cheers,
Ron

 

[Szabolcs Nagy]

with the current syntax L[i:i+1] returns [L], with nxlist it returns
L[i+1] if i<0.

L=range(10)
L[1:2]==[L[1]]==[1]
L[-2:-1]==[L[-2]]==[8]

L=nxlist(range(10))
L[1:2]==[L[1]]==[1]
L[-2:-1]==[L[-1]]==[9] # not [L[-2]]

IMHO in this case current list slicing is more consistent.

 

[Magnus Lycka]

Slicing is one of the best features of Python in my opinion, but
when you try to use negative index's and or negative step increments
it can be tricky and lead to unexpected results.

Hm... Just as with positive indexes, you just need to understand
the concept properly.

This topic has come up fairly often on comp.lang.python, and often
times, the responses include:

* Beginners should avoid negative extended slices.

* Slices with negative step increments are for advanced
python programmers.

I certainly wouldn't respond like that...

* It's not broke if you look at it in a different way.

* You should do it a different way.

Those are correct, but need to be complemented with helpful
explanations. It's often like this, that we think things seem
strange, because we don't see it right. We need a new perspective.
For instance, it took me a long time to understand OOP, but once
the concept was clear, things fell into place. Often, we fail to
see things due to preconceptions that mislead us. Like Yoda said:
"You need to unlearn what you have learnt."

- Extended slices with negative values return values that
have less items than currently.

- Slices with negative step values return entirely different
results.

Over my dead body! Honestly, I can't imagine you'll get agreement
over such a change.

REVERSE ORDER STEPPING
----------------------
When negative steps are used, a slice operation
does the following. (or the equivalent)

1. reverse the list
2. cut the reversed sequence using start and stop
3. iterate forward using the absolute value of step.

I think you are looking at this from the wrong perspective.

Whatever sign c has:
For s[a:b:c], a is the index for the first item to include,
b is the item after the last to include (just like .end() in
C++ iterators for instance), and c describes the step size.

To get a non-empty result, you obviously must have a > b iff
c < 0.

a defaults to 0, b defaults to None (which represents the
item beyond the end of the sequence) and c defaults to 1.

This is basically all you need to understand before you use
a or b < 0. There are no special cases or exceptions.

The concept of negative indices are completely orthogonal
to the concept of slicing today. You can learn and
understand them independently, and will automatically
be able to understand how to use the concepts together,
iff you actually understood both concepts correctly.

1. cut sequence using start and stop.
2 reverse the order of the results.
3. iterate forward using the absolute value of step.

With this solution, you suddenly have two different cases
to consider. You're suggesting that with c < 0, a should be
the end, and b should be the start. Now, it's no longer
obvious whether a or b should be excluded from the result.
I'm pretty sure the number of bugs and questions regarding
negative slices would grow quite a lot.

CURRENT INDEXING
----------------

Given a range [a,b,c]:

Positive indexing

| a | b | c |
+---+---+---+
0 1 2 3

Current negative indexing.

| a | b | c |
+---+---+---+
-3 -2 -1 -0

Almost right, there's no such thing as -0 in Python.
It's more like this:

> Current negative indexing.
>
> | a | b | c |
> +---+---+---+
> -3 -2 -1 None

Of course, [a,b,c][None] won't work, since it would
access beyond the list. It actually returns a TypeError
rather than an IndexError, and you might argue whether this
is the right thing to do. No big deal in my opinion. For
slices, None works as intended, giving the default in all
three positions. Thid means that if you want a negative
index that might be "-0" for the slice end, you simply
write something like

seq[start:end or None]

Accessing a range at the end of a list numerically
becomes inconvenient when negative index's are used
as the '-0'th position can not be specified numerically
with negative values.

But None works as is...

ONES BASED NEGATIVE INDEXING
----------------------------
Making negative index's Ones based, and selecting
individual item to the left of negative index's would enable
addressing the end of the list numerically.

Ones based negative index's.

| a | b | c |
+---+---+---+
-4 -3 -2 -1

Right. 0-based positive indices, and 1-based negative indices, but
you don't actually get the indexed value, but the one before...
I fear that your cure is worse then the "disease".

Today, a[x:x+1 or None] is always the same as [a[n]] for cases
where the latter doesn't yield an exception. With your change,
it won't be. That's not an improvement in my mind.

a[-3:-2] -> b # item between index's

But a[-3] -> a! Shrug!

The '~' is the binary not symbol which when used
with integers returns the two's compliment. This
works nice with indexing from the end of a list
because convieniently ~0 == -1.

But that's only helpful if you want to write literal sequence
indexes, and that's not how real programming works. Further,
it's only an advantage if we agree that your suggestion is
broken and we want some kind of half-baked fix, to make it
work as it should.

This creates a numerical symmetry between positive
indexing and '~' "End of list" indexing.

a[0] -> first item in the list.
a[~0] -> last item in the list.

You can use this silly trick today to, if you think
it looks prettier than a[-1], but for the newbie,
this either means that you introduce magic, or that
you must teach two concepts instead of one.

a[0:~0] -> whole list.

I don't see why that's better than a[:], or a[0:None].
After all, the visual aspect only appears when you type
literals, which isn't very interesting. For calculated
values on the slice borders, you still have -1 as end
value.

a[1:~1] -> center, one position from both ends.

This is just a convoluted way of writing a[1:-2], which
is exactly the same as you would write today.

 

[Scott David Daniels]

I think Ron's idea is taking off from my observation that if one's
complement, rather than negation, was used to specify measure-from-
right, we would have a simple consistent system (although I also
observed it is far too late to do that to Python now). Using such
a system should not define things as below:
Actually, the ~ operator is the one's complement operator.

> For calculated values on the slice borders, you still
> have -1 as end value.

But if you are defining the from-right as ones complement,
you use one's complement on the calculated values and
all proceeds happily. Since this could happen in Python,
perhaps we should call it Pythoñ.

a[1:~1] -> center, one position from both ends.

This is just a convoluted way of writing a[1:-2], which
is exactly the same as you would write today.

Actually, a[1 : -1] is how you get to drop the first and
last characters today. I suspect you knew this and were
just a bit in a hurry criticizing a lame-brained scheme.

-Scott David Daniels
(e-mail address removed)

 

[Ron Adam]

Hm... Just as with positive indexes, you just need to understand
the concept properly.



I certainly wouldn't respond like that...



Those are correct, but need to be complemented with helpful
explanations. It's often like this, that we think things seem
strange, because we don't see it right. We need a new perspective.
For instance, it took me a long time to understand OOP, but once
the concept was clear, things fell into place. Often, we fail to
see things due to preconceptions that mislead us. Like Yoda said:
"You need to unlearn what you have learnt."

To me, seeing a lot responses of this sort is an indicator it needs to
be simplified/or that something isn't right about them. They shouldn't
be that difficult to use or explain.

Over my dead body! Honestly, I can't imagine you'll get agreement
over such a change.

Yet, your description of how it works below is closer to what I'm
proposing than how they currently work.

REVERSE ORDER STEPPING
----------------------
When negative steps are used, a slice operation
does the following. (or the equivalent)

1. reverse the list
2. cut the reversed sequence using start and stop
3. iterate forward using the absolute value of step.

I think you are looking at this from the wrong perspective.

Whatever sign c has:
For s[a:b:c], a is the index for the first item to include,
b is the item after the last to include (just like .end() in
C++ iterators for instance), and c describes the step size.

Yes, and that is how it "should" work. But....

With current slicing and a negative step...

[ 1 2 3 4 5 6 7 8 9 ]
-9 -8 -7 -6 -5 -4 -3 -2 -1 -0

r[-3:] -> [7, 8, 9] # as expected
r[-3::-1] -> [7, 6, 5, 4, 3, 2, 1, 0] # surprise

The seven is include in both cases, so it's not a true inverse selection
either.

In most cases, negative step (or stride) values are used
to reverse the whole lists, so this issue doesn't come up.

To get a non-empty result, you obviously must have a > b iff
c < 0.

a defaults to 0, b defaults to None (which represents the
item beyond the end of the sequence) and c defaults to 1.

This is basically all you need to understand before you use
a or b < 0. There are no special cases or exceptions.

See above. ;-)

The concept of negative indices are completely orthogonal
to the concept of slicing today. You can learn and
understand them independently, and will automatically
be able to understand how to use the concepts together,
iff you actually understood both concepts correctly.

It's easy to think you understand something when you don't. I spend
quite a while figuring this out, And am sure about how it works. If
there are issues with this, then it will probably be in how I describe
it, what words or terminology is used, and weather or not it's the
proper approach.

There are actually two distinct proposals here, not just one.

1. Fixing negative strides so they return the slice indexed as you
say they should.

2. Using base one negative index's and picking item from the right
of negative index's instead of the right.


They don't both need to implemented, Item 1 could be fixed in 2.5.

Cheers,
Ron

 

[Steve Holden]

Magnus Lycka wrote: [...]
Actually, the ~ operator is the one's complement operator.

Actually the two are exactly the same thing. Could we argue about
substantive matters, please? ;-)

But if you are defining the from-right as ones complement,
you use one's complement on the calculated values and
all proceeds happily. Since this could happen in Python,
perhaps we should call it Pythoñ.

And how would that be pronounced? I understood that "ñ" would only
appear between two vowels.

a[1:~1] -> center, one position from both ends.

This is just a convoluted way of writing a[1:-2], which
is exactly the same as you would write today.

It does have the merit (if you think of it as a merit) of allowing
someone to write

a[n, ~n]

to remove n characters from each end of the string. Frankly I'd rather
deal with the Python that exists now than wrap my head around this
particular suggestion.

Actually, a[1 : -1] is how you get to drop the first and
last characters today. I suspect you knew this and were
just a bit in a hurry criticizing a lame-brained scheme.

Yes, I've been surprised how this thread has gone on and on.

regards
Steve

 

[Ron Adam]

I think Ron's idea is taking off from my observation that if one's
complement, rather than negation, was used to specify measure-from-
right, we would have a simple consistent system (although I also
observed it is far too late to do that to Python now). Using such
a system should not define things as below:

I didn't start with your observation in mind, but it ended up there
because the only way to extend the indexes past the last item is to not
use 0 along with negative index's. And once that's done the '~' forms
"just work" without any additional changes. ;-)

Yes, it may be too late for changing Python's built in indexing. And it
may also be that the majority may rather use it as is, rather than
change it to fix this edge case.

but rather use a form like:

Actually, the ~ operator is the one's complement operator.

Yes, thanks, my mistake.

For calculated values on the slice borders, you still
have -1 as end value.

But if you are defining the from-right as ones complement,
you use one's complement on the calculated values and
all proceeds happily. Since this could happen in Python,
perhaps we should call it Pythoñ.

a[1:~1] -> center, one position from both ends.

This is just a convoluted way of writing a[1:-2], which
is exactly the same as you would write today.

Actually, a[1 : -1] is how you get to drop the first and
last characters today. I suspect you knew this and were
just a bit in a hurry criticizing a lame-brained scheme.

-Scott David Daniels
(e-mail address removed)

 

[Fredrik Lundh]

Yes, I've been surprised how this thread has gone on and on.

it's of course a variation of

"You can lead an idiot to idioms, but you can't make him
think ;-)"

as long as you have people that insist that their original misunderstandings
are the only correct way to model the real world, and that all observed
inconsistencies in their models are caused by bugs in the real world, you'll
end up with threads like this.

</F>

 

[Ron Adam]

with the current syntax L[i:i+1] returns [L], with nxlist it returns
L[i+1] if i<0.

L=range(10)
L[1:2]==[L[1]]==[1]
L[-2:-1]==[L[-2]]==[8]

L=nxlist(range(10))
L[1:2]==[L[1]]==[1]
L[-2:-1]==[L[-1]]==[9] # not [L[-2]]

IMHO in this case current list slicing is more consistent.

Your second case should be:
L[-3:-2]==[L[-2]]==[8]

The single index is also the right side index of the range selection
ending in the same position just as with positive number the single
index is the left side index of a range starting in the same position.

So for positive indexing...
L[n1:n2]==[L[n1]]==first value of selection from left

and negative indexing...
L[n1:n2]==[L[n2]]==first value of selection from right


Currently for negative values...

L[n1:n2]==[L[n2-1]]==first value of selection from right


The symmetry is easier to see when using the '~' values.

LL=list(range(10))
Lx=nxlist(range(10))

LL[ 1: 2]==[LL[ 1]]==[1]
Lx[~2:~1]==[Lx[~1]]==[8]


Cheers,
Ron

 

[Ron Adam]

Steve Holden wrote:




it's of course a variation of

"You can lead an idiot to idioms, but you can't make him
think ;-)"

as long as you have people that insist that their original misunderstandings
are the only correct way to model the real world, and that all observed
inconsistencies in their models are caused by bugs in the real world, you'll
end up with threads like this.

</F>

Ok, -1 from both Fredrik and Steven

No problem,

But I think you are mistaken that I don't understand how slicing works.
(IMHO of course) ;-)


A more valid argument against this would be that more users may prefer
the current zero based, select to the right, indexing than have to
remember to subtract one from negative index's or use '~'.

I'm completely aware of that, and don't argue that that viewpoint is valid.

It also has backwards compatibility issues as well. Which is probably
enough in itself to kill it as a PEP.


However, I would like the inverse selection of negative strides to be
fixed if possible. If you could explain the current reason why it does
not return the reverse order of the selected range. I would appreciated it.

Cheers,
Ron

 

[Steve Holden]

Magnus Lycka wrote:




To me, seeing a lot responses of this sort is an indicator it needs to
be simplified/or that something isn't right about them. They shouldn't
be that difficult to use or explain.

It's a common misconception that all ideas should be explainable simply.
This is not necessarily the case, of course. When a subject is difficult
then all sorts of people bring their specific misconceptions to the
topic, and suggest that if only a few changes were made the whole thing
would be easier to understand.

Unfortunately, while that might make the topic in question easier to
understand for some it would make it difficult, and even
counter-intuitive, for others. As many have said before me, there's a
reason why slicing and indexing are the way they are. The interfaces
were designed by people with long years of programming and teaching
experience.

[...]
You said it quite well yourself:

It's easy to think you understand something when you don't. I spend
quite a while figuring this out, And am sure about how it works. If
there are issues with this, then it will probably be in how I describe
it, what words or terminology is used, and weather or not it's the
proper approach.

There are actually two distinct proposals here, not just one.

1. Fixing negative strides so they return the slice indexed as you
say they should.

2. Using base one negative index's and picking item from the right
of negative index's instead of the right.


They don't both need to implemented, Item 1 could be fixed in 2.5.

Given that Python has a 1's-complement operator already I don;t see why
you can't just leave Python alone and use it, since it seems to keep you
happy. If "fixing" item 1 in 2.5 would induce major code breakage,
there's less than a snowball's chance in hell it will happen.

"Professor Einstein, could you tell our readers how general relativity
works?"

regards
Steve

 

[Fredrik Lundh]

However, I would like the inverse selection of negative strides to be
fixed if possible. If you could explain the current reason why it does
not return the reverse order of the selected range.

why? you're not listening anyway.

</F>

 

[Paul Rubin]

Given that Python has a 1's-complement operator already I don;t see
why you can't just leave Python alone and use it,

What's the meaning of the 1's complement operator (for example, what
is ~1), when ints and longs are the same?

 

[Bengt Richter]

it's of course a variation of

"You can lead an idiot to idioms, but you can't make him
think ;-)"

as long as you have people that insist that their original misunderstandings
are the only correct way to model the real world, and that all observed
inconsistencies in their models are caused by bugs in the real world, you'll
end up with threads like this.

OTOH, ISTM we must be careful not to label an alternate alpha-version
"way to model the real world" as a "misunderstanding" just because it is alpha,
and bugs are apparent ;-)

BTW, it just occurred to me that alternate slice member semantics could be RELATIVE,
EACH depending on sign. I.e.,

x[start:stop:step]

could mean start with x[start] as now as a starting point reference point element,
then let stop be a relative range length in either direction from the
starting point, and step absolute value indicating step size, and its sign
indicating insertion side for zero length slices. In effect, stop would
be a count for the slice range in either direction starting with the starting
element

s = 'abcde'
s[2:2] => 'cd'
s[2:-2] => 'cb'
s[-2:-3] => 'dcb'
s[-2:0] => ''
s[2:0] => ''
s[-2:-3:2] => 'db'
r = range(10)
r[5:0] = 'a'
r => [0, 1, 2, 3, 4, 5, 'a', 6, 7, 8, 9]
r[-2:0:-1] = 'b'
r => [0, 1, 2, 3, 4, 5, 'a', 6, 7, 'b', 8, 9]
r[-2:0] = ['c', 'd']
r => [0, 1, 2, 3, 4, 5, 'a', 6, 7, 'b', 8, c, d, 9]

note that slice assignment would work by iterating through the right hand
sequence, which could do interesting things:

r = range(6)
r[3:-2] = 'ab'
r => [0, 1, 'b', 'a', 4, 5]
but two reverse relative slices match order, so
r = range(10)
r[5:-3] = range(10)[-1:-3] # rh seq is 9, 8
r => [0, 1, 8, 9, 4, 5]

I think this is kind of interesting, and I probably wouldn't have thought of
it if I had not been for Ron's enthusiasm for his "misunderstanding" ;-)

In a way, "misunderstandings" are the mutations of open source evolution of ideas,
most of which die, but some of which mutate again and may occasionally survive.
So we need them ;-)

Regards,
Bengt Richter

 

[Fredrik Lundh]

OTOH, ISTM we must be careful not to label an alternate alpha-version
"way to model the real world" as a "misunderstanding" just because it is alpha,
and bugs are apparent ;-)

in my post, "the real world" is the existing Python implementation. what
theoretical construct are you discussing?

</F>

 

[Steve Holden]

What's the meaning of the 1's complement operator (for example, what
is ~1), when ints and longs are the same?

Python 2.2.1 (#1, Aug 25 2004, 16:56:05)
[GCC 2.95.4 20011002 (Debian prerelease)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
$ python
Python 2.4.1 (#1, May 27 2005, 18:02:40)
[GCC 3.3.3 (cygwin special)] on cygwin
Type "help", "copyright", "credits" or "license" for more information.
What's going to change when ints and longs are finally integrated?

regards
Steve