小白一个，期末试题，哪位大神能帮帮忙

class Caesar:
 
    def __init__(self):
        a = list('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ ,.-!\'"')
        b = a[3:] + a[:3]
        self.emap = dict(zip(a,b))
        self.dmap = dict(zip(b,a))
 
    def encode(self, text):
        tmp = [ (x in self.emap and self.emap[x] or x) for x in text ]
        return ''.join(tmp)
 
    def decode(self, text):
        tmp = [ (x in self.dmap and self.dmap[x] or x) for x in text ]
        return ''.join(tmp)