[Python-ideas] Alternative weakref implementation

[MRAB]

Greg Ewing wrote:
> I've been thinking about an alternative implementation of weak
> references that would remove the limitation of only certain types
> being weakly referenceable.
> 
> This limitation can be a nuisance sometimes. The justification for it
> appears to be that you can't build a cycle exclusively out of objects
> that don't contain any mutable references, so there is no need to
> create weakrefs to such objects.
> 
> However, avoiding cycles is not the only reason for using weakrefs.
> They can also be useful for setting up callbacks to be triggered when
> an object is deallocated.
> 
> The restriction would be unnecessary if weakrefs could be implemented
> without incurring any overhead in the object being weakly referenced.
> This could be done by keeping a global dictionary, with keys that are
>  uncounted references to weakly referenced objects, and values that
> are lists of weakref objects.
> 
> Whenever an object is deallocated, the dict would be checked to see
> if there are any weak references to it, and if so they would be made
> dead and their callbacks called, and then the dict entry would be
> removed.
> 
> Can anyone see any serious flaws in this scheme?
> 
What would be the cost of checking for every deallocation? If most
objects are never weakly referenced, then could the check be made
cheaper by setting a flag in an object if a weak reference is ever made
to it?