Ka-Ping Yee <ping@lfw.org> writes:
> Hey, wait a second. To get l[start:end:step], you start with 'start'
> and keep adding 'step' until you get to -- but not including -- 'end',
> right? So l[len(l)-1:-1:-1] should return the entire list reversed,
> not the empty list.
Yes, but "-1" also means the last element of the list, so
>>> range(10)[3:-1:2]
[3, 5, 7]
If you want "to the end" behaviour, use None (or nothing):
>>> range(10)[5::-2]
[5, 3, 1]
>>> range(10)[5:None:-2]
[5, 3, 1]
(these two are indistinguishable)
Also, can I plead my case in that this was not my decision - it's the
logic implemented by PySlice_GetIndices.
> range() demonstrates the behaviour nicely:
>
> >>> size = 5
> >>> range(size-1,-1,-1)
> [4, 3, 2, 1, 0]
>
> Wouldn't l[sys.maxint:0:-1] return all but the first element of l in
> reverse order?
Yes.
> >>> range(size-1,0,-1)
> [4, 3, 2, 1]
>
> Note that we should deprecate the use of sys.maxint (preferring None)
> in this situation, since if the step is less than -1, the elements
> you get ought to depend on the value of sys.maxint % step. I'd expect:
>
> >>> l = [0, 1, 2, 3, 4, 5]
> >>> l[5:0:-3]
> [5, 2]
> >>> l[6:0:-3]
> [4, 1]
> >>> l[7:0:-3]
> [3, 0]
> >>> l[8:0:-3]
> [5, 2]
Whereas at the moment you get:
>>> l[5:0:-3]
[5, 2]
>>> l[6:0:-3]
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: slice indices must be integers
>>> l[7:0:-3]
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: slice indices must be integers
>>> l[8:0:-3]
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: slice indices must be integers
(which is definitely screwy). Perhaps I shouldn't use
PySlice_GetIndices (but then if I don't use it for this patch, what on
earth would you use it for?).
At any rate PySlice_GetIndices needs mending to accept longs (in the
style of PyObject_GetItem).
> hence
>
> >>> l[sys.maxint:0:-3]
> [???, ???] # depends on sys.maxint % 3
>
> My guiding principle is that the indices should always match what
> you get from range(), and if start or end are missing, you get the
> items you would have gotten if you extend the sequence out forever.
Aside from the interpretation of negative indices, I basically agree.
Cheers,
M.
--
languages shape the way we think, or don't.
-- Erik Naggum, comp.lang.lisp
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