creating garbage collectable objects (caching objects)

[Terry Reedy]

News123 wrote:
> Hi.
> 
> I started playing with PIL.
> 
> I'm performing operations on multiple images and would like compromise
> between speed and memory requirement.
> 
> The fast approach would load all images upfront and create then multiple
> result files. The problem is, that I do not have enough memory to load
> all files.
> 
> The slow approach is to load each potential source file only when it is
> needed and to release it immediately after (leaving it up to the gc to
> free memory when needed)
> 
> The question, that I have is whether there is any way to tell python,
> that certain objects could be garbage collected if needed and ask python
> at a later time whether the object has been collected so far (image has
> to be reloaded) or not (image would not have to be reloaded)

See the weakref module. But note that in CPython, objects are collected 
as soon as there all no normal references, not when 'needed'.
> 
> 
> # Fastest approach:
> imgs = {}
> for fname in all_image_files:
>     imgs[fname] = Image.open(fname)
> for creation_rule in all_creation_rules():
>     img = Image.new(...)
>     for img_file in creation_rule.input_files():
> 	img = do_somethingwith(img,imgs[img_file])
>     img.save()
> 
> 
> # Slowest approach:
> for creation_rule in all_creation_rules():
>     img = Image.new(...)
>     for img_file in creation_rule.input_files():
>         src_img = Image.open(img_file)
> 	img = do_somethingwith(img,src_img)
>     img.save()
> 
> 
> 
> # What I'd like to do is something like:
> imgs = GarbageCollectable_dict()
> for creation_rule in all_creation_rules():
>     img = Image.new(...)
>     for img_file in creation_rule.input_files():
>         if src_img in imgs: # if 'm lucke the object is still there
> 		src_img = imgs[img_file]
>         else:
>         	src_img = Image.open(img_file)
> 	img = do_somethingwith(img,src_img)
>     img.save()

> Is this possible?