[Python-3000] Spooky behavior of dict.items() and friends

[Guido van Rossum]

I'm not sure that it's really a safe vs. non-safe issue. The OP's
concern is that the change affects behavior of keys() and friends that
people have internalized for the past 18 years. I certainly don't see
this is a reason to change it back (we knew this would be the case). I
do think it needs to be ephasized in every "What's New" doc. It's
already in the list of Common Stumbling Blocks, but the paragraph
there could be expanded a bit.

--Guido

On Tue, Apr 1, 2008 at 3:53 PM, Benjamin Peterson
<musiccomposition at gmail.com> wrote:
>
>
>
>
> On Tue, Apr 1, 2008 at 5:25 PM, Paul Moore <p.f.moore at gmail.com> wrote:
> >
> > On 01/04/2008, "Martin v. Löwis" <martin at v.loewis.de> wrote:
> > > > What really bugs me about this state of affairs is that I consider the
> > >  > python 2 dict.items() to be safe and free of surprises, but I no
> > >  > longer feel the same way about it in 3; this is really about the fact
> > >  > that when you want to get the items, keys, or values of a dict, the
> > >  > simplest thing is no longer the safest thing. (I don't want to
> belabor
> > >  > this point too much since it isn't really my place to judge, but to
> > >  > me, dict views feel like they are a "special case that breaks the
> > >  > rules.")
> > >
> > >
> > > I feel to the contrary. 2.x .keys() was not safe, but 3.x keys() is.
> > >  When I iterate over the keys of a dictionary, I want all of them,
> > >  and I want only the keys. With 2.x, it could always happen that the
> > >  dictionary changes "behind me", and then I'd either iterate over
> > >  not all of the keys, or see some keys that aren't actually in the
> > >  dictionary anymore. With 3.x dictionary views, it's much safer now.
> >
> > The oddity with the 3.x keys() is that it's plausible to retain a
> > reference to d.keys(), but if you do so it can change if you alter d.
> > In 2.x, d.keys() is static and d.iterkeys() is (for all practical
> > purposes) not something you retain. The 3.x d.keys() "action at a
> > distance" is unfamiliar - I can't think of an example of this type of
> > view semantics in 2.x (at least in the core - numpy has had this for
> > some time, I believe).
> >
> > I suspect that view semantics will become less surprising over time,
> > but I think it's a fair point that it's something new to get used to.
> I personally find it less surprising. It seems logical to me that whatever
> you get from dict.items should reflect the current state of the dictionary.
> When you want it static, it's better to be explicit and say list(dict.keys
> or dict.values).
>
> >
> >
> > Paul.
> >
> >
> >
> >
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> >
>
>
>
> --
> Cheers,
> Benjamin Peterson
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-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)