Why is an iterable object not an iterator?

Question

Here is my code:

from collections import deque  class linehistory:     def __init__(self, lines, histlen=3):         self.lines = lines         self.history = deque(maxlen=histlen)      def __iter__(self):         for lineno, line in enumerate(self.lines,1):             self.history.append((lineno, line))             yield line      def clear(self):         self.history.clear()   f = open('somefile.txt') lines = linehistory(f) next(lines) 

Error:

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

I have no idea why the linehistory object is not an iterator since it has already included __iter__ method in the class.

Answer 1

The concept of iteration is well documented in the Python documentation.

In short, "iterable" is the object I want to iterate over, also called the container. This can be a list, a string, a tuple or anything else which consists of or can produce several items. It has __iter__() which returns an iterator.

An "iterator" is the object which is used for one iteration. It can be seen as a kind of "cursor". It has next() (in Python 2) or __next__() (in Python 3) which is called repeatedly until it raises a StopIteration exception. As any iterator is iterable as well (being its own iterator), it also has __iter__() which returns itself.

You can get an iterator for any iterable with iter(obj).

In your example, linehistory (which should be written LineHistory) is iterable as it has an .__iter__(). The generator object created with this is an iterator (as every generator object).