[Python-Dev] Allowing slicing of iterators

[Nick Coghlan]

Steven Bethard wrote:
> If we're really looking for a builtin, wouldn't it be better to go the
> route of getattr/setattr and have something like getslice that could
> operate on both lists and iterators?

Such a builtin should probably be getitem() rather than getslice() (since 
getitem(iterable, slice(start, stop, step)) covers the getslice() case). 
However, I don't really see the point of this, since "from itertools import 
islice" is nearly as good as such a builtin.

More importantly, I don't see how this alters Guido's basic criticism that 
slicing a list and slicing an iterator represent fundamentally different 
concepts. (ie. if "itr[x]" is unacceptable, I don't see how changing the 
spelling to "getitem(itr, x)" could make it any more acceptable).

If slicing is taken as representing random access to a data structure (which 
seems to be Guido's view), then using it to represent sequential access to an 
item in or region of an iterator is not appropriate.

I'm not sure how compatible that viewpoint is with wanting Python 3k to be as 
heavily iterator based as 2.x is list based, but that's an issue for the future.

For myself, I don't attach such specific semantics to slicing (I see it as 
highly dependent on the type of object being sliced), and consider it obvious 
syntactic sugar for the itertools islice operation. As mentioned in my previous 
message, I also think the iterator/iterable distinction should be able to be 
ignored as much as possible, and the lack of syntactic support for working with 
iterators is the major factor that throws the distinction into a programmer's 
face. It currently makes the fact that some builtins return lists and others 
iterators somewhat inconvenient.

Those arguments have already failed to persuade Guido though, so I guess the 
idea is dead for the moment (unless/until someone comes up with a convincing 
argument that I haven't thought of).

Given Guido's lack of enthusiasm for *this* idea though, I'm not even going to 
venture into the realms of "+" on iterators defaulting to itertools.chain or "*" 
to itertools.repeat.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at email.com   |   Brisbane, Australia
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