[Python-Dev] Weakref design questions

[Brian Quinlan]

David Abrahams:
> What's the point of the proxy? You never use it for anything. I guess
> you must've meant:

>     python_function(proxy)

> above.

<blush> Nice catch.

> Hum. In Boost.Python the default is to copy the C++ object when
> passing it to a Python callback, though it's possible for the user
> to explicitly say "just build a Python object around a reference to 
> the C++ object -- Python code beware of lifetime issues". I guess I 
> like your idea, though, as a safer alternative for non-copyable or 
> for very expensive-to-copy C++ objects.

DOMs, the data structures in question, can be obscenely expensive to
copy.

And I firmly believe that Python code should never be able to cause a
crash.

[nice demonstration deleted]
> How does this differ from what's happening for you?

When you created the bound method p.append, you caused the reference
count of p to be increased. 

In my case, since Python is not capable of altering the lifetime of p
(it's dead with the C library says that it is dead), this is a bad
thing.

So I proxy the bound methods and kill them when p dies.

> Hmm, it kinda seems like you want to prune some of them before the
> user's done with them. 

I actually presented two different desires at once and know I'm paying
the price. 

There are two reasons that I might want to delete a proxied object:

1. Its lifetime is over and I don't want anyone to access it
2. There are no proxy objects that refer to it, making it inaccessible
(since I hold the only actual reference)

Reason #1 is why I could like builtin methods to support weakrefs.
Reason #2 is why I would like a callback when the last weakref dies.

> Don't users expect objects to stay alive when they hold references?

Probably. But that is not always an option.

Cheers,
Brian