Bryan Olson <[EMAIL PROTECTED]> writes:
> The Python slice type has one method 'indices', and reportedly:
>
> This method takes a single integer argument /length/ and
> computes information about the extended slice that the slice
> object would describe if applied to a sequence of length
> items. It returns a tuple of three integers; respectively
> these are the /start/ and /stop/ indices and the /step/ or
> stride length of the slice. Missing or out-of-bounds indices
> are handled in a manner consistent with regular slices.
>
> http://docs.python.org/ref/types.html
>
>
> It behaves incorrectly
In some sense; it certainly does what I intended it to do.
> when step is negative and the slice includes the 0 index.
>
>
> class BuggerAll:
>
> def __init__(self, somelist):
> self.sequence = somelist[:]
>
> def __getitem__(self, key):
> if isinstance(key, slice):
> start, stop, step = key.indices(len(self.sequence))
> # print 'Slice says start, stop, step are:', start,
> stop, step
> return self.sequence[start : stop : step]
But if that's what you want to do with the slice object, just write
start, stop, step = key.start, key.stop, key.step
return self.sequence[start : stop : step]
or even
return self.sequence[key]
What the values returned from indices are for is to pass to the
range() function, more or less. They're not intended to be
interpreted in the way things passed to __getitem__ are.
(Well, _actually_ the main motivation for writing .indices() was to
use it in unittests...)
> print range(10) [None : None : -2]
> print BuggerAll(range(10))[None : None : -2]
>
>
> The above prints:
>
> [9, 7, 5, 3, 1]
> []
>
> Un-commenting the print statement in __getitem__ shows:
>
> Slice says start, stop, step are: 9 -1 -2
>
> The slice object seems to think that -1 is a valid exclusive
> bound,
It is, when you're doing arithmetic, which is what the client code to
PySlice_GetIndicesEx() which in turn is what indices() is a thin
wrapper of, does
> but when using it to actually slice, Python interprets negative
> numbers as an offset from the high end of the sequence.
>
> Good start-stop-step values are (9, None, -2), or (9, -11, -2),
> or (-1, -11, -2). The later two have the advantage of being
> consistend with the documented behavior of returning three
> integers.
I'm not going to change the behaviour. The docs probably aren't
especially clear, though.
Cheers,
mwh
--
(ps: don't feed the lawyers: they just lose their fear of humans)
-- Peter Wood, comp.lang.lisp
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