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

Steven Bethard steven.bethard at
Tue Jan 25 15:24:03 EST 2005

Michael Spencer wrote:
> Safe eval recipe posted to cookbook:

This recipe only evaluates constant expressions:

Evaluate constant expressions, including list, dict and tuple using the 
abstract syntax tree created by compiler.parse"

It means you can't eval arbitrary Python code -- it's basically just a 
data parser.  Handy in some situations, but not the equivalent of a 
limited Python virtual machine.

> Likewise, function calls are easily intercepted

I'm not sure I follow this...  How do you intend to intercept all 
function calls?

> As you say, attribute access to core functions appears to present the
> challenge. It is easy to intercept attribute access, harder to know
> what's safe.  If there were a known set of 'dangerous' objects e.g.,
> sys, file, os etc... then these could be checked by identity against any
> attribute returned

It sounds like you're suggesting overriding the global attribute access 
mechanism.  Is that right?  So that every time Python encountered an 
attribute access, you would verify that the attribute being accessed is 
not on the 'dangerous' list?  I don't know how to do that without 
basically rewriting some of Python's C code, though certainly I'm no 
expert in the area...

Also, I'm not sure identity is sufficient:

py> import sys
py> import new
py> new.module('newsys')
py> newsys = new.module('newsys')
py> newsys.__dict__.update(sys.__dict__)
py> newsys is sys
py> newsys == sys


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