If you need a function with arbitrary args you can use kwargs dictionary and exec statement over its items. If 'f' is not a builtin function you need to add from module import function in this auxiliar module.

arbitrary_args

def eval_fun(f, **args):
   ''' (str, dict) -> object
   f: Function as string.
   args: dictionary with arbitrary variables.
   Use arbitrary args in **args to eval general functions.
   Examples:
   >> eval_fun('x**2',x=5)
   25
   >> eval_fun('x*y', x=3, y=2)
   6
   '''
   for key in args.keys():
      exec(str(key)+'='+str(args[key]))
   return eval(f)