Python Language Iterables and Iterators Iterator vs Iterable vs Generator


Example

An iterable is an object that can return an iterator. Any object with state that has an __iter__ method and returns an iterator is an iterable. It may also be an object without state that implements a __getitem__ method. - The method can take indices (starting from zero) and raise an IndexError when the indices are no longer valid.

Python's str class is an example of a __getitem__ iterable.

An Iterator is an object that produces the next value in a sequence when you call next(*object*) on some object. Moreover, any object with a __next__ method is an iterator. An iterator raises StopIteration after exhausting the iterator and cannot be re-used at this point.

Iterable classes:

Iterable classes define an __iter__ and a __next__ method. Example of an iterable class :

class MyIterable:

    def __iter__(self):

         return self

    def __next__(self):
         #code

#Classic iterable object in older versions of python, __getitem__ is still supported...
class MySequence:

    def __getitem__(self, index):

         if (condition):
             raise IndexError
         return (item)

 #Can produce a plain `iterator` instance by using iter(MySequence())

Trying to instantiate the abstract class from the collections module to better see this.

Example:

Python 2.x2.3
import collections
>>> collections.Iterator()
>>> TypeError: Cant instantiate abstract class Iterator with abstract methods next
Python 3.x3.0
>>> TypeError: Cant instantiate abstract class Iterator with abstract methods __next__

Handle Python 3 compatibility for iterable classes in Python 2 by doing the following:

Python 2.x2.3
class MyIterable(object): #or collections.Iterator, which I'd recommend....

     ....

     def __iter__(self): 

          return self

     def next(self): #code

     __next__ = next

Both of these are now iterators and can be looped through:

ex1 = MyIterableClass()
ex2 = MySequence()

for (item) in (ex1): #code
for (item) in (ex2): #code

Generators are simple ways to create iterators. A generator is an iterator and an iterator is an iterable.