Safe Python Execution

[Graham]

I've been messing around with trying to get a small sandbox like
environment where i could execute python code in a "safe" way.
Basically what the old restricted execution module attempted to do.
I've written a small amount of code to get custom interpreter running,
but i'm not really sure if its safe.

The way i'm controlling functionality is with some games and exec, so
if 'code' was the text code you wanted to execute i run:

exec code in {'__builtins__':None"}

obviously this doesn't give you much to play with, but it does remove
file access and importing as far as i can tell. Can anyone think of a
hack around this? I assume if it was this easy it would be a module
already but i figured i would ask.

Graham.