PEP: Iterator

Classes can define how they are iterated over by defining an
    __iter__() method; this should take no additional arguments and
    return a valid iterator object.  A class that wants to be an
    iterator should implement two methods: a next() method that behaves
    as described above, and an __iter__() method that returns self.

一个类如果想要有和Iterator一样的行为，需要实现两个基本函数：__iter__()和next()，前者返回类对象self本身，后者返回容器的下一个元素，如果到底最后一个元素，需要抛出异常StopIteration。

下面是关于Dictionary类的相关例子：

This means that we can write

          if k in dict: ...

      which is equivalent to

          if dict.has_key(k): ...

这两个表达式的意义是一样的。

This means that we can write

          for k in dict: ...

      which is equivalent to, but much faster than

          for k in dict.keys(): ...

这两个表达式的意义是一样的。

          for key in dict.iterkeys(): ...

          for value in dict.itervalues(): ...

          for key, value in dict.iteritems(): ...

这三个表达式分别显示的返回了三种Iterator。