Python Iterators

Iterators in Python are used to iterate over a group of elements, containers, like list. For a container to support iterator, it must provide __iter__().

container.__iter__() :
This returns an iterator object.

Iterator protocol:

The iterator object is required to support the iterator protocol. Iterator protocol is implemented by an iterator object by providing definition of the following 2 functions:

1. iterator.__iter__() :
    It returns the iterator object itself. This is required to allow both containers and iterators to be used with the for and in statements.

2. iterator.__next__() :
    It returns the next item from the container. If there are no further items, raise the StopIteration exception.
 

Example of iterator on list:

>>> a=[1,2,3]
>>> i1= a.__iter__()   # creating an iterator using __iter__() on container
>>> i1         
<listiterator object at 0x7f5192ccbd10>
>>> i2= iter(a)        # creating another iterator using iter() which calls __iter__() on container
>>> i2
<listiterator object at 0x7f5192ccbcd0>
>>> i1.next()
1
>>> next(i1)           # calls i1.next()
2
>>> next(i2)
1

Iterators are required to implement __iter__ which returns the iterator (self) . Hence it can be used with for in

>>> for x in i1:
...   print x
...
3