实现一个ordeeddict

class MyOrderdict():
    def __init__(self, mydict):
        self._cur = 0
        self._mykeys = []
        self._myvalues = []
        self._mydict = mydict
        for i in mydict:
            self._mykeys.append(i[0])
            self._myvalues.append(i[1])

    def keys(self):
        return self._mykeys

    def values(self):
        return self._myvalues

    def items(self):
        return self._mydict

    def __getitem__(self, item):
        if item not in self._mykeys:
            raise ValueError
        return self._myvalues[self._mykeys.index(item)]

    def __setitem__(self, key, value):
        self._mykeys.append(key)
        self._myvalues.append(value)

    def __getattr__(self, item):
        if item not in self._mykeys:
            raise ValueError
        return self._myvalues[self._mykeys.index(item)]

    def __iter__(self):
        return self

    def __next__(self):
        self._cur += 1
        if self._cur > len(self._mykeys):
            self._cur = 0
            raise StopIteration
        return (self._mykeys[self._cur - 1], self._myvalues[self._cur - 1])

    def __len__(self):
        return len(self._mykeys)

    def __str__(self):
        return str(self._mydict)

a = MyOrderdict([('a', 1), ('b', 2)])
a.a
a['a']
a.keys()
a.values()
len(a)
for i in a:
　　print(i)
for i in a.items():
　　print(i)