[Python-Dev] Dict constructor

[Guido van Rossum]

> I want a set module, but if I finish Greg's abandoned work I want sets of
> sets too.  Sets don't have "keys", they're conceptually collections of
> values, and it would be as odd not to allow sets containing sets as not to
> allow lists containing lists, or to ban dicts as dict values.

IMO it's no odder than disallowing dicts as dict keys: it's a hack
that allows a much faster implementation.

> That is, like sets of sets in Icon too, this is a notion of inclusion by
> object identity (although Icon does that on purpose, while the BTree-based
> set mostly inherits it from that BTrees don't implement any comparison
> slots).  That's very easy to implement.  It's braindead if you think of sets
> as collections of values, but that's what taking pain too seriously leads
> to.

I don't think it is acceptable to have sets-of-sets but test for
membership (in that case) by object identity.

If you really think object identity is all that's needed, I suggest we
stick to disallowing sets of sets; algorithms needing
sets-of-set-object-identities can use id() on the inner sets.

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