Runs the user-supplied progress calculation rule

def _eval_progress(self, match):
        '''
        Runs the user-supplied progress calculation rule
        '''
        _locals = {k: safe_float(v) for k, v in match.groupdict().items()}
        if "x" not in _locals:
            _locals["x"] = [safe_float(x) for x in match.groups()]
        try:
            return int(eval(self.progress_expr, {}, _locals))
        except:
            return None