Iterables and Iterators

Iterator vs Iterable vs Generator

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 - The method can take indices (starting from zero) and raise an IndexError when the indices are no longer valid.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:

import collections
>>> collections.Iterator()
>>> TypeError: Cant instantiate abstract class Iterator with abstract methods next

>>> TypeError: Cant instantiate abstract class Iterator with abstract methods __next__

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

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.

What can be iterable

Iterating over entire iterable

Verify only one element in iterable

Extract values one by one

Iterator isn’t reentrant!