Simple and safe evaluator

[Simon Forman]

On Jun 11, 1:25 pm, bvdp <b... at mellowood.ca> wrote:
> Is there a simple/safe expression evaluator I can use in a python
> program. I just want to pass along a string in the form "1 + 44 / 3" or
> perhaps "1 + (-4.3*5)" and get a numeric result.
>
> I can do this with eval() but I really don't want to subject my users to
> the problems with that method.
>
> In this use I don't need python to worry about complex numbers,
> variables or anything else. Just do the math on a set of values. Would
> eval() with some restricted list of permitted operators do the trick?
>
> I'm feeling too lazy to write/debug my own parser for this :)
>
> Thanks, Bob.



Funny, I need exactly the same kind of parser myself right now.
Fredrik Lundh has posted some code-and-explanation on an excellent
simple parser that's easy to extend.  http://effbot.org/zone/simple-iterator-parser.htm

Just make it recognize the operator tokens you're interested in and if
a string parsers w/o errors then you know it's safe to eval().

I probably won't get to writing this myself for a few days or a week,
but if you do will you post it here (or send me a copy)?  I'll do the
same if I get to it sooner.

Regards,
~Simon