On Wed, Oct 06, 2010 at 05:28:13PM -0400, Terry Reedy wrote:
> On 10/6/2010 7:14 AM, Antoon Pardon wrote:
>
> >>That right-hand-half-open intervals (i.e. a<= i< b, equivalently [a,
> >>b) ), which are what Python uses, are to be preferred.
> >>(See aforelinked PDF: http://www.cs.utexas.edu/users/EWD/ewd08xx/EWD831.PDF)
>
> This specifically discusses subsequences of 'natural numbers'.
Sure, but I don't think we should limit ourselves to subsequence of
natural numbers. It is my impression that orginally slices were
also limited to indicating subsequences of natural numbers. This
original limitation, has guided its semantics that resulted in
what we have now,
> >The problem is that the slice notation is sometimes handy in situations where
> >an open interval doesn't allow easily to mark what you want.
> >
> >For instance I have at one time implemted a Tree. This is a dict like
> >structure
> >but it allows to visit the keys in order. Because there is an order, slice
> >notation can make sense. e.g. if T is a tree with names as keys,
> >T['bea':'mike']
> >is a subtree where we have for each key that 'bea'<= key< 'mike'.
>
> >But what if I wanted a subtree where 'mike' was still included, but nothing
> >further?
> >Or what if the keys were floats or tuples and I wanted an inclusive upper
> >boundary?
>
> Strings and tuples are not natural numbers, but do have least
> members ('' and ()), so the bottom end had better be closed.
Why? The fact that we have a bottom element in the item space,
doesn't imply that the sequence I need is easiest defined by
using an inclusive lower limit. What if I wanted all none-empty
strings/tuples keys in the tree? The easiest way to specify that
would be by saying the key needed to be larger than the empty
string/tuple. If the keys were strings I could fudge it by starting
with chr(0), but what value should use as start, if I wanted all
non-empty tuples?
> Since
> substracting strings and tuples in meaningless, the argument of
> having stop-start == number of items does not apply. Floats can be
> subtracted, but the result in not a count of included items. So
> write the .__getitem__ method of *your* class how it best fits your
> use-cases.
My use cases depend on the specific problem I need to solve. Sometimes
the problem is easiest solved with an inclusive limit, sometimes it
is with an exclusive limit. The slice notation although at first sight
a natural way to specify the boundaries, seems on closer inspection
to be too limited.
> Just be aware that an inclusive upper boundary means that
> s[a:b]+s[b:c] will no longer be s[a:c] because b will be duplicated.
> Let me also point out integer slices for builtins are adjusted to
> that they refer to actual slice positions. This is harder to do with
> strings;-). Also, suppose you want all strings beginning with 'a' or
> 'b'. With an open upper end, Tree['a':'c'] will do that. With closed
> upper end, you would need
> Tree['a':'bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb'] or somesuch.
Yes I know. I'm starting to think about aproaching this in a different
manner.
> >And what if you needed the reverse sequence. If you start with inclusive
> >limit,
> >the reverse of a<= item<= b is b>= item>= a. If the second limit is to be
> >exclusive the reverse of a<= item< b becomes (b - 1)>= item> (a - 1).
>
> Slices with positive strides can be defined and understood in terms
> of slice positions before the first item, between all successive
> pairs of items and after the last. By this same definition and
> understanding, s[b:a:-1] should be reversed(s[a:b]), which is what
> many expect. It is not because the definition is given in terms of
> the translation into indexes and blindly applied to the case of
> negative strides without the needed adjustment. I consider this a
> design mistake, at least for many uses. (Extended slices, if not
> extended slicing, were added for numerical Python and they may have
> wanted the current definition for negative strides.)
I agree that this is a design mistake. Personnaly I find it horrible
that in the following expression: L[a:b:-1], it is impossible to
give a numeric value to b, that will include L[0] into the reversed
slice. It is the reason why I avoid reversed slices as much as
possible.
--
Antoon Pardon
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