Interpreting a Python string as a conditional statement?

Question

I\'m processing some json-formatted log files in python. It\'s very straightforward to code some conditional queries, e.g.

line=[1,\'runtime\',{\'elapsed\':         


        

Answer 1

There is not really a safe way to do that. For basic conditionals you could parse an input string of a specific format. If your input was in the format "var > 5" you could parse it like this:

var, op, num = argv[1].split()
var = getattr(sys.modules[__name__], var) # Get a reference to the data
num = int(num)
if op == ">":
   r = var > num
elif op == "<":
   r = var < num
...

if r:
    

To support more complicated statements you would need to improve the parser. If you don't trust your input you should wrap the getattr and int in try/except blocks. To support int or float or another var you would need quite a bit of logic.