Re: [pygame] Referencing instances by memory address

[éæå <onpon4@xxxxxxxxx>]

I didn't know about weak references. They're never mentioned in any books or tutorials I've looked at. I guess I'll look into them.

On Mon, Jul 12, 2010 at 8:57 PM, Brian Fisher <brian@xxxxxxxxxxxxxxxxxxx> wrote:

You still haven't explained why referencing things by id is better for your needs than actually just referencing the object. Python's weakref module is perfect for this kind of thing, you can make a reference to an object that is fine for self-references (won't create cycles) and even cooler, the reference will become invalid when the object is deleted, so you'll never have the problem where the id somebody has stored has become invalid (or even worse, now points to a new object) because the original object was deleted and the guy with the id reference didn't know.

so what I'm saying is like instead of doing stuff like this:
ÂÂÂ self.other_bullet_id = id(other_bullet)
and this:
ÂÂÂ other_bullet = global_dictionary[self.other_bullet_id]
ÂÂÂ other_bullet.do_stuff()

you'd do stuff like this:
ÂÂÂ self.other_bullet_ref = weakref.ref(other_bullet)
and this:
ÂÂÂ other_bullet = self.other_bullet_ref()
ÂÂÂ other_bullet.do_stuff()

and the cool part about that, is that "other_bullet" will be None, if the bullet object was deleted, when using weakrefs, so you can catch and handle that case just fine.

While with id's, the id may have been reused and now refer to the wrong object, or will just simply be an invalid id when looking it up in the dictionary. Also, you'll never be sure if you were completely diligent in maintaining that dictionary of id's - so you won't know if the id is now invalid cause the bullet was deleted, or if you had a bug in the code that maintains the dict.

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basically, by trying to reference things by id, you've made a system that operates like using C/C++ pointers, with most (but not all) of the problems of such a system. Meanwhile, python gives you a ref-counted garbage collected system with weak references that solves all those problems, and is most likely much more efficient than your pointer re-implementation.

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so my question is the same - why are you doing this id system? What problems does it solve for you that weak references wouldn't?

On Mon, Jul 12, 2010 at 4:09 PM, éæå <onpon4@xxxxxxxxx> wrote:

(I ask because it seems like doing what you are doing would be a big ol' error prone hassle and waste of your time compared to other more commonly used techniques in python, but I can't figure out exactly what problem that dictionary approach was originally conceived of as a solution for)

Simple: I needed instances to be able to reference themselves, and I didn't feel like generating my own unique IDs and passing them on as a parameter. :)

Why are you storing stuff by id() in a dictionary in the first place? When would you want to look something up by it's id() when you wouldn't want just to have a reference (possible a weakref) to the object itself?