Question

I want to define a class that supports __getitem__, but does not allow iteration. for example:

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

cb = B()

for x in cb:
   print x

在<代码>B上,我可以添加什么内容,使<代码>在cb中的x:失效?

Answer 1

我认为,一种稍微更好的解决办法是提出一种类型错误,而不是一种简单的例外情况(通常情况下是非可转让类别的情况):

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

Answer 2

在对

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

友情链接