[Python-Dev] Proposal: defaultdict

[Raymond Hettinger]

[Terry Reedy]
> One is a 'universal dict' that maps every key to something -- the default if 
> nothing else.  That should not have the default ever explicitly entered. 
> Udict.keys() should only give the keys *not* mapped to the universal value.

Would you consider it a mapping invariant that "k in dd" implies "k in 
dd.keys()"?

Is the notion of __contains__ at odds with notion of universality?


Raymond