Private data

[Dustan]

On Mar 18, 7:06 am, Steven D'Aprano
<s... at REMOVE.THIS.cybersource.com.au> 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