Question

I am studying Alex Marteli’s Python in a Nutshell and the book suggests that any object that has a next() method is (or at least can be used as) an iterator. It also suggests that most iterators are built by implicit or explicit calls to a method called iter.

After reading this in the book, I felt the urge to try it. I fired up a python 2.7.3 interpreter and did this:

>>> x = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> for number in range(0, 10):
...     print x.next()

However the result was this:

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
AttributeError: 'list' object has no attribute 'next'

In confusion, I tried to study the structure of the x object via dir(x) and I noticed that it had a __iter__ function object. So I figured out that it can be used as an iterator, so long as it supports that type of interface.

So when I tried again, this time slightly differently, attempting to do this:

>>> _temp_iter = next(x)

I got this error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: list object is not an iterator

But how can a list NOT be an iterator, since it appears to support this interface, and can be certainly used as one in the following context:

>>> for number in x:
...     print x

Could someone help me clarify this in my mind?

Answer 1

They are iterable, but they are not iterators. They can be passed to iter() to get an iterator for them either implicitly (e.g. via for) or explicitly, but they are not iterators in and of themselves.