[Python-ideas] add a list.swap() method

Kristján Valur Jónsson kristjan at ccpgames.com
Wed Jun 24 18:26:55 CEST 2009

I agree that as presented, it is a bit radical due to the concerns raised by Raymond and others.
But I still think it is a useful recipe for those that know what they are doing, and then perhaps as the standalone function, SwapLists(a,b) like we implemented in-house for our purposes.
I wonder, where do such recipes belong?


> -----Original Message-----
> From: python-ideas-bounces+kristjan=ccpgames.com at python.org
> [mailto:python-ideas-bounces+kristjan=ccpgames.com at python.org] On
> Behalf Of Aahz
> Sent: 23. júní 2009 23:15
> To: python-ideas at python.org
> Subject: Re: [Python-ideas] add a list.swap() method
> On Tue, Jun 23, 2009, Raymond Hettinger wrote:
> >
> > [Kristj?n Valur J?nsson]
> >>
> >> The idea is to speed up the swapping of list elemenst so that
> >>     a.swap(b)
> >> is equivalent to
> >>    a[:], b[:] = b[:], a[:]
> >> but without all the overhead of creating slices, assigning them and
> so forth.
> >
> > The problem with the OP's example is that it only makes sense in
> > interactions between a list and a list subclass that won't be broken
> > by it.  For straight list-to-list interactions, it is better to write
> > "a,b = b,a" (though this is not exactly the same thing since the
> > identity of a and b will change, not just their contents).  For list
> > subclass uses (a more advanced topic), some example will work and
> some
> > won't (I gave two failing examples in the tracker discussion).  When
> > the list subclass is a C extension written by a third-party, it may
> > not be possible to know whether or not a swap will break invariants.
> > That's a killer.
> Thanks for the detailed response and explanation of why Kristjan wants
> the feature and why it isn't the same thing as the standard tuple swap.
> Your assessment makes sense, and I agree that this doesn't belong as a
> standard list feature.

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