Simple and safe evaluator

[bvdp]

I'm finding my quest for a safe eval() quite frustrating :)

Any comments on this: Just forget about getting python to do this and, 
instead, grab my set of values (from a user supplied text file) and call 
an external program like 'bc' to do the dirty work. I think that this 
would avoid someone from embedding os.system("rm ...") in what I thought 
would be a math expression and having it maybe do damage? Perhaps I'm 
getting too paranoid in my old age.

I guess this would slow things down a bit, but that is not a big 
concern. Bigger concern would be that I'm not sure if 'bc' or whatever 
is guaranteed to be on other platforms than *nix. And if I want to be 
really paranoid, I could worry that someone had planted a bad 'bc' on 
the target.