Handling slices

[Alex Martelli]

Michael Hudson wrote:

> Francois Pinard <pinard at iro.umontreal.ca> writes:
> 
>> Hi, Python friends.
>> 
>> I'm currently using Python 2.2.1.  How does one support slices when given
>> to __getitem__?  Suppose I get a slice object.  How can I use it to get
>> a slice of a local array?  In other words, how one does apply a slice?
> 
> In 2.2.1, by hand, I think.

Essentially, yes, though it can be nicer than this...:

> def __getitem__(self, item):
>     if isinstance(item, types.SliceType):
>        if item.step != 1:
>           raise ValueError
>        if item.start is None:
>           start = 0
>        else:
>           start = item.start
>        if item.stop is None:
>           stop = len(self.local_array)
>        else:
>           stop = item.stop
>        return self.local_array[start:stop]

I'd probably take another tack, such as (warning, untested!!!)...:

def applyslice(asequence, aslice):
    seqlen = len(asequence)
    def mkidx(x, default):
        if x is None: return default
        elif x>=0: result = x
        else: result = x + seqlen
        return max(0, min(x, seqlen-1))
    if aslice.step is None: step = 1
    else: step = aslice.step
    if step > 0:
        default_start = 0
        default_stop = seqlen
    else:
        default_start = seqlen-1
        default_stop = -1
    start = mkidx(aslice.start, default_start)
    stop = mkidx(aslice.stop, default_stop)
    return type(asequence)([asequence[z]
            for x in xrange(start, stop, step)
           ])

of course, this needs to be refactored (and how!) to also
be usable for __setitem__ AND __delitem__... and it sure
makes a LOT of sense to specialcase the common case where
step is 1 to delegate right to asequence's slicing...


> I presume you don't need to be told that this is all much nicer in
> 2.3...

Given that built-in sequences now accept slice objects in their
__getitem__, all of the above becomes just a
    return asequence[aslice]
which IS indeed nicer;-).  But you might still need to keep the
applyslice function around in case you need to deal with sequence
types from older extensions, or the like...


Alex