Python基础教程（第二版）例子程序__getitem__函数

if not isinstance(key, (int,long)  ):#type(key) == int:#ss

def checkIndex(key):
    '''
    为了能被接受，键应该是一个非负整数，如果不是一个整数，会引发error；
    如果是复数，引发IndexError
    '''
    if not isinstance(key, int):#type(key) == int:#ss
        raise TypeError
    if key <0:
        raise indexError
class ArithmeticSequence(object):
    def __init__(self,start=0,step=1):
        self.start = start
        self.step = step
        self.changed = {}
    def __getitem__(self, key):
        '''
        Get an item from the arithmetic sequence.
        '''
        checkIndex(key)
        try:
            return self.changed[key]
        except KeyError:
            return self.start + key*self.step #...计算值)
    def __setitem__(self, key,value):
        '''
        修改算数序列中的一个项
        '''
        checkIndex(key)
        self.changed[key] = value # 保存更改后的值
    def __iter__(self):
        return self
class Fib(object):
    def __getitem__(self, n):
        a, b = 1, 1
        for x in range(n):
            a, b = b, a + b
        return a
s = ArithmeticSequence(1,2)
print(s[4])
s = Fib()
print(s[2])