[Python-ideas] Why don't CPython strings implement slicing using a view?

[Neil Girdhar]

Exactly.

You know, it might be nice to have a recipe that creates a view to any
abc.Sequence for when you know that the underlying sequence won't change
(or don't care).  Something like:

class View: ...

some_view = View("some string", slice(2, 5))

some_view[0: 2]
"me"

etc.

Also a MutableView class could be used for abc.MutableSequences.

Best,

Neil

On Sat, May 9, 2015 at 1:06 AM, Chris Angelico <rosuav at gmail.com> wrote:

> On Sat, May 9, 2015 at 2:04 PM, Nikolaus Rath <Nikolaus at rath.org> wrote:
> > On May 07 2015, Steven D'Aprano <steve-iDnA/YwAAsAk+I/
> owrrOrA at public.gmane.org> wrote:
> >> But a view would be harmful in this situation:
> >>
> >> s = "some string"*1000000
> >> t = s[1:2]  # a view maskerading as a new string
> >> del s
> >>
> >> Now we keep the entire string alive long after it is needed.
> >>
> >> How would you solve the first problem without introducing the second?
> >
> > Keep track of the reference count of the underlying string, and if it
> > goes down to one, turn the view into a copy and remove the sliced
> > original?
>
> Oops, mis-sent (stupid touchpad on this new laptop). Trying again.
>
> There might be multiple views, so a hard-coded refcount-of-one check
> wouldn't work. The view would need to keep a weak reference to its
> underlying string - but not in the sense of the Python weakref module,
> which doesn't seem to have any notion of "about to be garbage
> collected", but only "has now been garbage collected". Notably, by the
> time a callback gets called, it's too late to retrieve information
> from the callback itself. A modified form of weakref could do it,
> though; with the understanding that the referents are immutable, and
> premature transform from view to coalesced slice has no consequence
> beyond performance, this could be done.
>
> Ideally, it'd be an entirely invisible optimization.
>
> ChrisA
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