Can `weakref` callbacks replace `__del__`?

Question

Is there any obstacle that prevents weakref from doing everything that __del__ does but with much stronger guarantees (e.g., finalize guarantees that the call will be made before the interpreter exits, and the order of calls is well-defined, etc.)?

It seems that in the distant past it was thought that weakref would eventually lead to the removal of __del__ from the language.

What prevented this from happening?

There seems to be few use cases for __del__, and all the ones I'm aware of seem to work at least as well (and usually much better) with weakref callbacks or weakref.finalize.

Update:

With PEP 442 dramatically improving the behavior of __del__, and the concerns with weakref mentioned by @gz and @user2357112, I'm wondering if the language is generally moving towards making __del__ more reliable, or towards using weakref instead of __del__, or both.

Answer 1

There's a somewhat pragmatic reason __del__ is still around. Several signficant weakref improvements, including finalize, were new in Python 3.4. So, replacing __del__ with better weakrefs missed the window for language breaking changes with py3k.

I think most uses can be replaced by the base weakref functionality, but I'm struck by this observation from Richard Oudkerk in issue 15528 where proposed and implemented finalize:

[Weakref callbacks] are low level, and working out how to use them correctly requires a bit of head scratching. One must find somewhere to store the weakref till after the referent is dead, and without accidentally keeping the referent alive. Then one must ensure that the callback frees the weakref (without leaving any remnant ref-cycles).

When it is an option, using a __del__ method is far less hassle.

Anyway, perhaps the question should be brought up again when Python 4 is being considered? ;)