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
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:
0, 1, 2, 3, 4, 5, 6, 7, 8, 9] for number in range(0, 10): print x.next()x = [
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:
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?
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.
You need to convert list to an iterator first using
In : x = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] In : it=iter(x) In : for i in range(10): it.next() ....: ....: Out: 0 Out: 1 Out: 2 Out: 3 Out: 4 Out: 5 Out: 6 Out: 7 Out: 8 Out: 9 In : 'next' in dir(it) Out: True In : 'next' in dir(x) Out: False
checking whether an object is iterator or not:
In : isinstance(x,collections.Iterator) Out: False In : isinstance(x,collections.Iterable) Out: True In : isinstance(it,collections.Iterable) Out: True In : isinstance(it,collections.Iterator) Out: True
Just in case you are confused about what the difference between iterables and iterators is. An iterator is an object representing a stream of data. It implements the iterator protocol:
Repeated calls to
the iterator’s next() method return successive items in the stream. When
no more data is available the iterator object is exhausted and any further calls to its
next() method just raise StopIteration again.
On the other side iterable objects implement the
__iter__ method that when called returns an iterator, which allows for multiple passes over their data. Iterable objects are reusable, once exhausted they can be iterated over again. They can be converted to iterators using the
So if you have a list (iterable) you can do:
1,2,3,4] for i in l: print i, 1 2 3 4 for i in l: print i, 1 2 3 4l = [
If you convert your list into an iterator:
# equivalent to iter(l) for i in il: print i, 1 2 3 4 for i in il: print i,il = l.__iter__()
List is not iterator but list contains an iterator object
__iter__ so when you try to use for loop on any list, for loop calls
__iter__ method and gets the iterator object and then it uses next() method of list.
x = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] it = x.__iter__()
it contains iterator object of
x which you can use as
it.next() until StopIteration exception is thrown