在python中,如何使类支持getitem,但不允许迭代?

我认为一个稍微好一点的解决方案是引发一个类型错误,而不是一个简单的异常(这是非iterable类通常会发生的情况:

class A(object):
    # show what happens with a non-iterable class with no __getitem__
    pass

class B(object):
    def __getitem__(self, k):
        return k
    def __iter__(self):
        raise TypeError('%r object is not iterable'
                        % self.__class__.__name__)

测试:

>>> iter(A())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'A' object is not iterable
>>> iter(B())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "iter.py", line 9, in __iter__
    % self.__class__.__name__)
TypeError: 'B' object is not iterable

从这个问题的答案 question 我们可以看到,如果存在的话,将在getitem之前调用iter,所以简单地将b定义为:

class B:
   def __getitem__(self, k):
      return k

   def __iter__(self):
      raise Exception("This class is not iterable")

然后:

cb = B()
for x in cb: # this will throw an exception when __iter__ is called.
  print x