creating garbage collectable objects (caching objects)

[Gabriel Genellina]

En Mon, 29 Jun 2009 08:01:20 -0300, Dave Angel <davea at ieee.org> escribió:
> News123 wrote:

>> What I was more concerned is a group of output images depending on TWO
>> or more input images.
>>
>> Depending on the platform (and the images) I might not be able to
>> preload all two (or more images)
>>
>> So,  as CPython's garbage collection takes always place immediately,
>> then I'd like to pursue something else.
>> I can create a cache, which caches input files as long as python leaves
>> at least n MB available for the rest of the system.

> As I said earlier, I think weakref is probably what you need.  A weakref  
> is still a reference from the point of view of the ref-counting, but not  
> from the point of view of the garbage collector.  Have you read the help  
> on weakref module?  In particular, did you read Pep 0205?    
> http://www.python.org/dev/peps/pep-0205/

You've misunderstood something. A weakref is NOT "a reference from the  
point of view of the ref-counting", it adds zero to the reference count.  
When the last "real" reference to some object is lost, the object is  
destroyed, even if there exist weak references to it. That's the whole  
point of a weak reference. The garbage collector isn't directly related.

py> from sys import getrefcount as rc
py> class X(object): pass
...
py> x=X()
py> rc(x)
2
py> y=x
py> rc(x)
3
py> import weakref
py> r=weakref.ref(x)
py> r
<weakref at 00BE56C0; to 'X' at 00BE4F30>
py> rc(x)
3
py> del y
py> rc(x)
2
py> del x
py> r
<weakref at 00BE56C0; dead>

(remember that getrefcount -as any function- holds a temporary reference  
to its argument, so the number it returns is one more than the expected  
value)

> Object cache is one of the two reasons for the weakref module.

...when you don't want the object to stay artificially alive just because  
it's referenced in the cache. But the OP wants a different behavior, it  
seems. A standard dictionary where images are removed when they're no more  
needed (or a memory restriction is fired).

-- 
Gabriel Genellina