[Python-Dev] Proposal: defaultdict

[Raymond Hettinger]

> > Also, I think has_key/in should return True if there is a default.

> It certainly seems desirable to see True where d[some_key]
> doesn't raise an exception, but one could argue either way.

Some things can be agreed by everyone:

* if __contains__ always returns True, then it is a useless feature (since 
scripts containing a line such as "if k in dd" can always eliminate that line 
without affecting the algorithm).

* if defaultdicts are supposed to be drop-in dict substitutes, then having
__contains__ always return True will violate basic dict invariants:
   del d[some_key]
   assert some_key not in d


Raymond