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I understand.  Ideally, *all* methods would respond to a 
single overridden method, but I think this is just a fact of 
life in object oriented programming.

I can't remember where you gave an example of a 
d.__getitem__() subclass override, but you were careful to 
point out that other methods, like d.get() also needed to 
be overridden so that the modified access applied 
everywhere.  Likewise, __iter__() or any other object 
access method must be assumed to access the underlying 
data structure directly and must be overridden.   For 
instance, creating a dictionary with case insensitive 
lookups entails overriding __getitem__(k), get(k,default), 
and pop(k) -- no one of them can be presumed to inform 
the others.