Re: creating garbage collectable objects (caching objects)

En Mon, 29 Jun 2009 08:01:20 -0300, Dave Angel <davea@xxxxxxxx> 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

.

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