limited python virtual machine (WAS: Another scripting language implemented into Python itself?)

[Steven Bethard]

Fuzzyman wrote:
 > Cameron Laird wrote:
 > [snip..]
 >
 >>This is a serious issue.
 >>
 >>It's also one that brings Tcl, mentioned several
 >>times in this thread, back into focus.  Tcl presents
 >>the notion of "safe interpreter", that is, a sub-
 >>ordinate virtual machine which can interpret only
 >>specific commands.  It's a thrillingly powerful and
 >>correct solution to the main problem Jeff and others
 >>have described.
 >
 > A better (and of course *vastly* more powerful but unfortunately only
 > a dream ;-) is a similarly limited python virutal machine.....

Yeah, I think there are a lot of people out there who would like 
something like this, but it's not quite clear how to go about it.  If 
you search Google Groups, there are a lot of examples of how you can use 
Python's object introspection to retrieve "unsafe" functions.

I wish there was a way to, say, exec something with no builtins and with 
import disabled, so you would have to specify all the available 
bindings, e.g.:

     exec user_code in dict(ClassA=ClassA, ClassB=ClassB)

but I suspect that even this wouldn't really solve the problem, because 
you can do things like:

py> class ClassA(object):
...     pass
...
py> object, = ClassA.__bases__
py> object
<type 'object'>
py> int = object.__subclasses__()[2]
py> int
<type 'int'>

so you can retrieve a lot of the builtins.  I don't know how to retrieve 
  __import__ this way, but as soon as you figure that out, you can then 
do pretty much anything you want to.

Steve