[Python-Dev] Improved evaluator added to ast module

[Daniel Holth]

On Thu, Oct 11, 2012 at 1:36 PM, Vinay Sajip <vinay_sajip at yahoo.co.uk> wrote:
> Daniel Holth <dholth <at> gmail.com> writes:
>
>> How does this compare to the markerlib approach? In markerlib you just
>> make sure all the AST nodes are in a set of allowed nodes, currently
>> (Compare, BoolOp, Attribute, Name, Load, Str, cmpop, boolop), and then
>> use the normal eval(). Is one way more secure / fast / flexible than
>> the other?
>
> I don't think performance is an issue, and the markerlib approach seems just
> as reasonable as the one I've taken, except that it calls eval(), whereas my
> approach doesn't. It boils down to what should be allowed in expressions, and
> what shouldn't be.
>
> ISTM there is a space for a limited evaluator that's less limiting than
> literal_eval(). I do realise that this type of sandboxing is not easy to achieve,
> and I'm not aiming to advance the state of the art here - I just want to close
> the issue in the best way I can.

I bet the literal_eval approach simply predates compile(ast) which is
a Python 2.6 feature. It is also probably slightly faster on CPython
to avoid compile(ast) if you are only evaluating the code once.