What exactly are iterator, iterable, and iteration

Question

What are the most basic definitions of "iterable", "iterator" and "iteration in Python?

I've read multiple definitions but their exact meaning still won't sink in.

Can someone please help me with the basic idea?

Answer 1

Iteration is a general term for taking each item of something, one after another. Any time you use a loop, explicit or implicit, to go over a group of items, that is iteration.

In Python, iterable and iterator have specific meanings.

An iterable is an object that has an __iter__ method which returns an iterator, or which defines a __getitem__ method that can take sequential indexes starting from zero (and raises an IndexError when the indexes are no longer valid). So an iterable is an object that you can get an iterator from.

An iterator is an object with a next (Python 2) or __next__ (Python 3) method.

Whenever you use a for loop, or map, or a list comprehension, etc. in Python, the next method is called automatically to get each item from the iterator, thus going through the process of iteration.

A good place to start learning would be the iterators section of the tutorial and the iterator types section of the standard types page. After you understand the basics, try the iterators section of the Functional Programming HOWTO.

Answer 2

Here's the explanation I use in teaching Python classes:

An ITERABLE is:

• anything that can be looped over (i.e. you can loop over a string or file) or
• anything that can appear on the right-side of a for-loop: for x in iterable: ... or
• anything you can call with iter() that will return an ITERATOR: iter(obj) or
• an object that defines __iter__ that returns a fresh ITERATOR, or it may have a __getitem__ method suitable for indexed lookup.

An ITERATOR is an object:

• with state that remembers where it is during iteration,
• with a __next__ method that:
  • returns the next value in the iteration
  • updates the state to point at the next value
  • signals when it is done by raising StopIteration
• and that is self-iterable (meaning that it has an __iter__ method that returns self).

Notes:

• The __next__ method in Python 3 is spelt next in Python 2, and
• The builtin function next() calls that method on the object passed to it.

For example:

>>> s = 'cat'      # s is an ITERABLE
                   # s is a str object that is immutable
                   # s has no state
                   # s has a __getitem__() method 

>>> t = iter(s)    # t is an ITERATOR
                   # t has state (it starts by pointing at the "c"
                   # t has a next() method and an __iter__() method

>>> next(t)        # the next() function returns the next value and advances the state
'c'
>>> next(t)        # the next() function returns the next value and advances
'a'
>>> next(t)        # the next() function returns the next value and advances
't'
>>> next(t)        # next() raises StopIteration to signal that iteration is complete
Traceback (most recent call last):
...
StopIteration

>>> iter(t) is t   # the iterator is self-iterable

Answer 3

Iterable:- something that is iterable is iterable; like sequences like lists ,strings etc. Also it has either the __getItem__() method or an iter() function which returns an iterator.

Iterator:- When we get iterator object from the iter() method of iterable; we call __next__() method (in python3) or simply next() (in python2) to get elements one by one. This class or instance of this class is called an iterator.

From docs:-

The use of iterators pervades and unifies Python. Behind the scenes, the for statement calls iter() on the container object. The function returns an iterator object that defines the method __next__() which accesses elements in the container one at a time. When there are no more elements, __next__() raises a StopIteration exception which tells the for loop to terminate. You can call the __next__() method using the next() built-in function; this example shows how it all works:

>>> s = 'abc'
>>> it = iter(s)
>>> it
<iterator object at 0x00A1DB50>
>>> next(it)
'a'
>>> next(it)
'b'
>>> next(it)
'c'
>>> next(it)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    next(it)
StopIteration

Ex of a class:-

class Reverse:
    """Iterator for looping over a sequence backwards."""
    def __init__(self, data):
        self.data = data
        self.index = len(data)
    def __iter__(self):
        return self
    def __next__(self):
        if self.index == 0:
            raise StopIteration
        self.index = self.index - 1
        return self.data[self.index]


>>> rev = Reverse('spam')
>>> iter(rev)
<__main__.Reverse object at 0x00A1DB50>
>>> for char in rev:
...     print(char)
...
m
a
p
s

Answer 4

iterable = [1, 2] 

iterator = iter(iterable)

print(iterator.__next__())   

print(iterator.__next__())   

so,

1. iterable is an object that can be looped over. e.g. list , string , tuple etc.

2. using the iter function on our iterable object will return an iterator object.

3. now this iterator object has method named __next__ (in Python 3, or just next in Python 2) by which you can access each element of iterable.

so, OUTPUT OF ABOVE CODE WILL BE:

1

2

Answer 5

Iterables have a __iter__ method that instantiates a new iterator every time.

Iterators implement a __next__ method that returns individual items, and a __iter__ method that returns self .

Therefore, iterators are also iterable, but iterables are not iterators.

Luciano Ramalho, Fluent Python.

Answer 6

Before dealing with the iterables and iterator the major factor that decide the iterable and iterator is sequence

Sequence:Sequence is the collection of data

Iterable:Iterable are the sequence type object that support Iter method.

Iter method:Iter method take sequence as an input and create an object which is known as iterator

Iterator:Iterator are the object which call next method and transverse through the sequence.On calling the next method it returns the object that it transversed currently.

example:

x=[1,2,3,4]

x is a sequence which consists of collection of data

y=iter(x)

On calling iter(x) it returns a iterator only when the x object has iter method otherwise it raise an exception.If it returns iterator then y is assign like this:

y=[1,2,3,4]

As y is a iterator hence it support next() method

On calling next method it returns the individual elements of the list one by one.

After returning the last element of the sequence if we again call the next method it raise an StopIteration error

example:

>>> y.next()
1
>>> y.next()
2
>>> y.next()
3
>>> y.next()
4
>>> y.next()
StopIteration