Iterables and Iterators

Iterables and Iterators are different types of objects involved in python programming. Learn more about Iterables and Iterators and generator here.

Difference between generator, Iterables and Iterators

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 – The method can take indices (starting from zero) and raise an IndexError when the indices are no longer valid.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.x Version ≥ 2.3

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

Iterables and Iterators: Python 3.x Version ≥ 3.0

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.x Version ≥ 2.3

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.

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

Iterables and Iterators: 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]

Iterables and Iterators: 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

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()

Learn more

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