[Python-Dev] defaultdict and on_missing()

[Raymond Hettinger]

[Guido van Rossum"]
> If we removed on_missing() from dict, we'd have to override
> __getitem__ in defaultdict (regardless of whether we give
>defaultdict an on_missing() hook or in-line it).

You have another option.  Keep your current modifications to
dict.__getitem__ but do not include dict.on_missing().  Let it only
be called in a subclass IF it is defined; otherwise, raise KeyError.

That keeps me happy since the basic dict API won't show on_missing(),
but it still allows a user to attach an on_missing method to a dict subclass 
when
or if needed.  I think all your test cases would still pass without 
modification.
This is approach is not much different than for other magic methods which
kick-in if defined or revert to a default behavior if not.

My core concern is to keep the dict API clean as a whistle.


Raymond