Iterables and Iterators

Extract values one by one

Start with iter() built-in to get iterator over iterable and use next() to get elements one by one until StopIteration is raised signifying the end:

s = {1, 2}   # or list or generator or even iterator
i = iter(s)  # get iterator
a = next(i)  # a = 1
b = next(i)  # b = 2
c = next(i)  # raises StopIteration

Iterating over entire iterable

s = {1, 2, 3}

# get every element in s
for a in s:
    print a  # prints 1, then 2, then 3

# copy into list
l1 = list(s)  # l1 = [1, 2, 3]

# use list comprehension
l2 = [a * 2 for a in s if a > 2]  # l2 = [6]

Iterator isn't reentrant!

def gen():
    yield 1

iterable = gen()
for a in iterable:
    print a

# What was the first item of iterable? No way to get it now.
# Only to get a new iterator
gen()

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 - The method can take indices (starting from zero) and raise an IndexError when the indices are no longer valid.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

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.

Verify only one element in iterable

Use unpacking to extract the first element and ensure it's the only one:

a, = iterable

def foo():
    yield 1

a, = foo()  # a = 1

nums = [1, 2, 3]
a, = nums  # ValueError: too many values to unpack

What can be iterable

Iterable can be anything for which items are received one by one, forward only. Built-in Python collections are iterable:

[1, 2, 3]     # list, iterate over items
(1, 2, 3)     # tuple
{1, 2, 3}     # set
{1: 2, 3: 4}  # dict, iterate over keys

Generators return iterables:

def foo():  # foo isn't iterable yet...
    yield 1

res = foo()  # ...but res already is