Questions about weakref

[Thomas Jollans]

On 2018-10-16 11:10, Frank Millman wrote:
> 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.

This sounds reasonable, though what you probably really want is a
signal/slot library that does all of this for you, like PySignal.

I believe all the pure-Python signal/slot libraries use weakrefs internally.

-- Thomas