Negative array indicies and slice()
ethan at stoneleaf.us
Thu Nov 1 15:12:15 CET 2012
Andrew Robinson wrote:
> On 10/31/2012 02:20 PM, Ian Kelly wrote:
>> On Wed, Oct 31, 2012 at 7:42 AM, Andrew Robinson wrote:
>>> Then; I'd note: The non-goofy purpose of slice is to hold three
>>> data values; They are either numbers or None. These *normally*
>>> encountered values can't create a memory loop.
>>> So, FOR AS LONG, as the object representing slice does not contain
>>> an explicit GC pair; I move that we mandate (yes, in the current
>>> python implementation, even as a *fix*) that its named members may
>>> not be assigned any objects other than None or numbers....
>>> eg: Lists would be forbidden....
>>> Since functions, and subclasses, can be test evaluated by int(
>>> the_thing_to_try ) and * can too,
>>> generality need not be lost for generating nothing or numbers.
>> PEP 357 requires that anything implementing the __index__ special
>> method be allowed for slicing sequences (and also that __index__ be
>> used for the conversion). For the most part, that includes ints and
>> numpy integer types, but other code could be doing esoteric things
>> with it.
> I missed something... (but then that's why we're still talking about it...)
> Reading the PEP, it notes that *only* integers (or longs) are permitted
> in slice syntax.
Keep in mind that PEPs represent Python /at that time/ -- as Python
moves forward, PEPs are not updated (this has gotten me a couple times).
>> The change would be backward-incompatible in any case, since there is
>> certainly code out there that uses non-numeric slices -- one example
>> has already been given in this thread.
> Now, I'm thinking -- The purpose of index(), specifically, is to notify
> when something which is not an integer may be used as an index; You've
> helpfully noted that index() also *converts* those objects into numbers.
> Ethan Fullman mentioned that he used the names of fields, "instead of
> having to remember the _offsets_"; Which means that his values _do
> convert_ to offset numbers
Furman, actually. :)
And my values do *not* convert to indices (at least, not automatically).
My __getitem__ code looks like:
elif isinstance(item, slice):
sequence = 
if isinstance(item.start, (str, unicode)) \
or isinstance(item.stop, (str, unicode)):
field_names = dbf.field_names(self)
start, stop, step = item.start, item.stop, item.step
if start not in field_names or stop not in field_names:
"Either %r or %r (or both) are not valid field names"
% (start, stop))
if step is not None and not isinstance(step, (int, long)):
"step value must be an int or long, not %r"
start = field_names.index(start)
stop = field_names.index(stop) + 1
item = slice(start, stop, step)
for index in self._meta.fields[item]:
In other words, the slice contains the strings, and my code calculates
the offsets -- Python doesn't do it for me.
> His example was actually given in slice syntax notation [::].
> Hence, his objects must have an index() method, correct?.
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