How to process user supplied formulas?

Question

I have a dictionary containing a set of key values available through a web application: I want to process user supplied formulas like: ((value1+value3)/value4)*100

W

Answer 1

Thanks for all the input. I personally found that lacopo's answer suites my situation best.

Here's a rough idea of the solution:

import sys

values={'one':10,'two':1245,'three':674365,'four':65432,'five':131}
print str(values)

formula=raw_input('Please enter formula:')

for key, val in values.items():
        formula = formula.replace(key, str(val))

whitelist=[ '+','-','/','*','^','(',')' ]

to_evaluate=re.findall('\D',formula)
to_evaluate=list(set(to_evaluate))

for element in to_evaluate:
        if not element in whitelist:
                print "Formula contains an invalid character: "+str(element)
                sys.exit(1)


print eval(formula)