Questions about weakref

[Frank Millman]

Hi all

I have some questions about using weakrefs.

My first question is whether weakrefs are the correct tool for my situation. 
My use-case is not mentioned in the docs, so maybe it is not intended to be 
used this way.

I have a lot of objects active in my app. Some of them (A) are fairly 
long-lived. Others (B) are short-lived, but hold a reference to one of the A 
objects. For the lifetime of the existence of B, it needs to be notified of 
any change of state of A. Therefore when B is created, it adds itself to a 
list maintained by A. It adds a tuple consisting of itself and a function 
(C). Whenever A detects a change of state, it iterates through the list and 
calls the function C, passing B as an argument.

I have quite a bit of additional housekeeping to perform to ensure that, 
when B goes out of scope, it removes itself from the list in A. I was hoping 
to use weakrefs to make this automatic. So the first question is, is this a 
reasonable use of weakrefs?

If yes, my second question is how to guard against A calling methods using 
dead B objects, as A has no control over when B might be gc'd.

My thinking is to use a WeakKeyDictionary, with the B object as the key and 
the C function as the data.

When A detects a change of state, instead of iterating over the keys, it 
should call keyrefs() to get the weakrefs and iterate over that. For each 
weakref it should 'call' it to get the actual key, and check for None to 
ensure that it is still alive. Does that sound safe?

Incidentally, the docs are slightly misleading. It says that the method 
keyrefs returns 'an iterable of the weak references to the keys'. I was not 
sure if that would expose me to the 'change size while iterating' error, so 
I checked the source. The source says 'return list(self.data)', and the 
docstring clearly states 'Return a list of weak references to the keys'. I 
think the docs should say the same. Should I raise an issue for this?

Frank Millman