Weakref.ref callbacks and eliminating __del__ methods

[Mike C. Fletcher]

I'm looking at rewriting parts of Twisted and TwistedSNMP to eliminate
__del__ methods (and the memory leaks they create). Looking at the docs
for 2.3's weakref.ref, there's no mention of whether the callbacks are
held with a strong reference. My experiments suggest they are not...
i.e. I'm trying to use this pattern:

class Closer( object ):
"""Close the OIDStore (without a __del__)"""
def __init__( self, btree ):
"""Initialise the closer object"""
self.btree = btree
def __call__( self, oldObject=None ):
"""Regular call via self.close or weakref deref"""
if self.btree:
self.btree.close()
self.btree = None
class BSDOIDStore(oidstore.OIDStore):
def __init__( self, filename, OIDs = None ):
"""Initialise the storage with appropriate OIDs"""
self.btree = self.open( filename )
self.update( OIDs )
self.close = Closer( self.btree )
weakref.ref( self, self.close )

but the self.close reference in the instance is going away *before* the
object is called.

So, this approach doesn't *seem* to work (the Closer doesn't get
called), so I can gather that the callbacks don't get incref'd (or they
get decref'd during object deletion).

I can work around it in this particular case by defining a __del__ on
the Closer, but that just fixes this particular instance (and leaves
just as many __del__'s hanging around). I'm wondering if there's a
ready recipe that can *always* replace a __del__'s operation?

I know I heard a rumour somewhere about Uncle Timmy wanting to eliminate
__del__ in 2.5 or thereabouts, so I gather there must be *some* way of
handling the problem generally. The thing is, weakref callbacks trigger
*after* the object is deconstructed, while __del__ triggers before...
must be something clever I'm missing.

Throw an old doggie a bone?
Mike

________________________________________________
Mike C. Fletcher
Designer, VR Plumber, Coder
http://www.vrplumber.com
http://blog.vrplumber.com

 

[Alex Martelli]

weakref.ref( self, self.close )

but the self.close reference in the instance is going away *before* the
object is called.

Uh -- what's holding on to this weakref.ref instance? I guess the
weakreference _itself_ is going away right after being created...


Alex

 

[Mike C. Fletcher]

Uh -- what's holding on to this weakref.ref instance? I guess the
weakreference _itself_ is going away right after being created...

You know, you're right. I'd been thinking (not-very-clearly) that
registering the callback would keep the reference alive until it was
called, guess I'm too used to PyDispatcher's operation. Urgh, that's
seriously annoying, requires storing the callback somewhere external.

Back to __del__ I suppose.

Thanks Alex,
Mike


________________________________________________
Mike C. Fletcher
Designer, VR Plumber, Coder
http://www.vrplumber.com
http://blog.vrplumber.com