[Python-3000] PEP: Eliminate __del__

[Adam Olsen]

On 5/4/07, Guido van Rossum <guido at python.org> wrote:
> On 5/4/07, Steven Bethard <steven.bethard at gmail.com> wrote:
> > On 5/4/07, Guido van Rossum <guido at python.org> wrote:
> > > Hm, a thought just occurred to me. Why not arrange for object.__new__
> > > to call [the moral equivalent of] weakref.cleanup(self,
> > > self.__class__.__del__), and get rid of the direct call to __del__
> > > from the destructor? (And the special-casing of objects with __del__
> > > in the GC module, of course.)
> >
> > That seems like a good idea, though I'm still a little unclear as to
> > how far the AttrMap should be going to look like a real instance. As
> > it stands, you can only access items from the instance __dict__. That
> > means no methods, class attributes, etc.::
>
> Oh, you mean 'self' as passed to the callback is not the instance?
> That kills the whole idea (since the typical __del__ calls
> self.flush() or self.close()).
>
[..snip example using __dict__..]
>
> If it really has to be done this way, I think the whole PEP is doomed.

Any attempt that keeps the entire contents of __dict__ alive is
doomed.  It's likely to contain a cycle back to the original object,
and avoiding that is the whole point of jumping through these hoops.

I've got a metaclass that moves explicitly marked attributes and
methods into a "core" object, allowing you to write code like this:

class MyFile(safedel):
    __coreattrs__ = ['_fd']
    def __init__(self, path):
        super(MyFile, self).__init__()
        self._fd = os.open(path, ...)
    @coremethod
    def __safedel__(core):
        core.close()
    @coremethod
    def close(core):
        # This method is written to be idempotent
        if core._fd is not None:
            os.close(core._fd)
            core._fd = None

I've submitted it to the python cookbook, but I don't know how long
it'll take to get posted; it's a little on the long side at 163 lines.

The biggest limitation is you can't easily use super() in core
methods, although the proposed changes to super() would probably fix
this.

-- 
Adam Olsen, aka Rhamphoryncus