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I wouldn't want to use WeakKeyDictionary directly for this, 
since I'm using it in a fairly time-critical place in C++ 
code. I could do the same thing with a proper subtype of 
dictionary using C++ code, and that's fine if you have a 
guarantee that a custodian has only one ward. Otherwise you 
need to use a collection of wards as the value, which again 
costs an extra allocation for the common case where a 
custodian really does have only one ward. Yes, it would be 
amortized over the number of wards for any custodian, but 
as I say the common case will have one ward per custodian. 
So I'd still really like to have weakref subclassing.