On Mar 18, 7:06 am, Steven D'Aprano <[EMAIL PROTECTED]> wrote: > On Sat, 17 Mar 2007 09:31:01 -0700, Dustan wrote: > >http://dustangroups.googlepages.com/privateattributesinpython > > > This is something that I just threw together this morning, after a > > eureka moment. It's a way of creating private class attributes and > > static function variables (I'm not 100% sure if that's the correct > > terminology, but you get what I mean). I haven't tried to create > > private instance attributes, mainly because it would just be too > > difficult, and it would be awful syntax. I'm not considering actually > > using this, but I do have a couple questions about it. > > > 1. Has anyone else ever come up with something like this? I can't > > imagine I'm the only person who's ever thought of this. > > I've never seen anything like this before, but then I haven't gone looking > for anything like this. > > > 2. Is it possible to hack into something like this? ie, would it be > > possible to see and change these variables from client code (assuming > > the data manager has been properly removed from sight, as shown on the > > last line of class block TestPrivateClassAttributes)? > > Yes. > > First, an example of the code in action. > > >>> import PrivateAttributes > >>> obj = PrivateAttributes.TestPrivateClassAttributes() > >>> obj.getNumInstances() > 1 > >>> another = PrivateAttributes.TestPrivateClassAttributes() > >>> obj.getNumInstances() > 2 > >>> athird = PrivateAttributes.TestPrivateClassAttributes() > >>> athird.getNumInstances() > > 3 > > The getNumInstances method reports the number of instances of the > PrivateAttributes class. There's no obvious class attribute where this > count is being kept: > > >>> obj.__class__.__dict__.keys() > > ['__module__', 'getNumInstances', '__dict__', '__weakref__', '__doc__', > '__init__'] > > Here's how to hack it, and make it report wrong numbers. > > >>> c = obj.getNumInstances.func_closure > >>> c[1].cell_contents.numInstances = -300 > > >>> athird.getNumInstances() > -300 > >>> afourth = PrivateAttributes.TestPrivateClassAttributes() > >>> athird.getNumInstances() > > -299 > > So yes, it is absolutely hackable.
I did have a feeling that it was hackable, but had no idea how one could possibly go about hacking it (I was starting to wonder of there was a way to apply locals() and globals() on functions). But now I (ehem) sorta know how it's done. > Now, I'm hardly a Python guru, but in about fifteen minutes I followed the > trail through the object chain, and found how to hack this. An real guru > would probably do it in three minutes. > > I was helped a bit by having the source code. But even without the source > code, I reckon I could have done it in an hour or so, if I was motivated > enough. All the tools you need are a Python interactive session, the dir() > function and the dis module. I have used all of those before, but I haven't been able to fully understand the output of the dis module; maybe that's my problem. > -- > Steven -- http://mail.python.org/mailman/listinfo/python-list
