[Python-Dev] Dict constructor

[Guido van Rossum]

> > This would run into similar problems as the PEP's auto-freeze approach
> > when using "s1 in s2".  If s1 is a mutable set, this creates an
> > immutable copy for the test and then throws it away.  The PEP's
> > problem is that it's too easy to accidentally freeze a set; the
> > problem with your proposal is "merely" one of performance.  Yet I
> > think both are undesirable, although I still prefer your solution.
> 
> If performance is a problem (and I can well see it might be!) then
> Set.__contains__(self, x) needs to use a specialized version of
> the ad-hoc adaptation code I proposed for insertion:
> 
> >     def insert(self, item):
> >         try: item = item.asSetItem()
> >         except AttributeError: pass
> >         self.data[item] = True
> 
> One possible route to such optimization is to introduce another
> class, called _TemporarilyImmutableSet, able to wrap a MutableSet x,
> have the same hash value that x would have if x were immutable, and
> compare == to whatever x compares == to.
> 
> Set would then expose a private method _asTemporarilyImmutable.
> ImmutableSet._asTemporarilyImmutable would just return self;
> MutableSet._asTemporarilyImmutable would return
> _TemporarlyImmutableSet(self).
> 
> Then:
> 
>     class Set(object):
>         ...
>         def __contains__(self, item):
>             try: item = item._asTemporarilyImmutable()
>             except AttributeError: pass
>             return item in self.data

Sounds reasonable.  Who's gonna do an implementation?  There's Greg
Wilson's version, and there's an alternative by Aric Coady
<coady@bent-arrow.com> that could be used as a comparison.

--Guido van Rossum (home page: http://www.python.org/~guido/)