[Python-Dev] Dict constructor

[Guido van Rossum]

> What about the following compromise: there are two set types,
> ImmutableSet and MutableSet, with a common supertype Set.
> ImmutableSet adds __hash__, while MutableSet adds insert and remove,
> to the common core of methods inherited from Set, such as
> __contains__ and __iter__.

Reasonable.

> Since it's apparently too wild an idea to say "adapt to protocol" when one
> means "adapt to protocol", at least for the next few releases (and that, in
> the optimistic hypothesis that my future rewrite of the adaptation PEP is
> favorably received), there will of course need to arise yet another special
> purpose way to express this same general idea, such as:
> 
> 
> class MutableSet(Set):
>     ...
>     def insert(self, item):
>         try: item = item.asSetItem()
>         except AttributeError: pass
>         self.data[item] = True
> 
>     def asSetItem(self):
>         return ImmutableSet(self)
> 
> 
> or the like.  

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.

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