python class design: directly use member vars in methods or pass member vars to methods?

Just curious, what are the pros and cons of the following patterns?

pattern 1

class Myclass:
    def __init__(self):
        self.val = "foo"
        self.process()
    def process(self):
        #do_sth on self.val

pattern 2

class Myclass:
    def __init__(self):
        self.val = "foo"
        self.process(self.val)
    def process(self, val):
        #do_sth on val

Which one is preferable, and why?

Thanks!!!