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. 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. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
