[Python-ideas] A way to tell the gc it's OK to delete an object with a __del__

[MRAB]

On 2014-11-13 18:18, Chris Barker wrote:
> I've been poking into reference counting, circular references, etc,
> trying to understand how the garbage collector works.
>
> In practice, I'm trying to figure out a way to get a couple modules I
> work with not to leak memory: one is an extension to wxPython that I
> wrote, that creates a circular reference involving wx objects -- these,
> as is often the case with wrappers, have a custom __del__ method.
>
> The other is the python netcdf4 lib -- which has a circular reference
> built in, so that Datasets (which map to a file on disk) know about
> Variables (which map to data arrays within the file) and vice versa. We
> tried using weak references here, but it turns out that some users like
> to work with Variables without a reference to the Dataset, so that
> didn't work.
>
> Anyway, in my poking around, I think I understand that the gc won't
> delete "unreachable" objects if the have a custom __del__ method,
> because __del__ methods often handle resources that may depend on other
> objects, and bad (really bad) things can happen if they are deleted out
> of order, and the gc has no idea what order to delete.
>
> However -- some __del__ methods are perfectly safe regardless of delete
> order.
>
> So it would be nice to be able to somehow let the gc know that a
> particular object is safe to delete at any time.
>
> Is there a technical reason this can't be done? It seems something as
> simple as a __delete_in_any_order__ attribute would do it. Yes, this is
> dangerous, but authors of such extension objects need to know what they
> are doing anyway.
>
> Is there a technical complication I'm not thinking of?
>
> It seems if we could pull this off, we could eliminate one lasting ugly
> wart in python memory management.
>
There's some code in this that you might find useful:

http://permalink.gmane.org/gmane.comp.python.ideas/20334