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.. "auto_examples/basics/iterator_vs_iterable.py"
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.. _sphx_glr_auto_examples_basics_iterator_vs_iterable.py:


============================
1.16 iterator vs iterable
============================
.. important::
  This lesson is still under development.

.. GENERATED FROM PYTHON SOURCE LINES 11-15

An ``iterable`` is any object which can be looped over. An
``iterable`` can be turned into ``iterator`` after applying ``iter`` function on
it. An ``iterator`` is an object upon which ``next`` method can be applied.
(Note: This is not a complete definition but without OOP, I suppose this will work)

.. GENERATED FROM PYTHON SOURCE LINES 17-22

.. code-block:: default


    provinces = ["Sindh", "Balochistan", "Janubi Pubjab", "Hazara", "Pakhtunkhwa"]
    for prov in provinces:
        print(prov)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    Sindh
    Balochistan
    Janubi Pubjab
    Hazara
    Pakhtunkhwa




.. GENERATED FROM PYTHON SOURCE LINES 23-27

.. code-block:: default


    # uncomment following line
    # next(provinces)  # -> TypeError: 'list' object is not an iterator








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As the error explicitly states, ``provinces`` is not ``iterator``. so we can
not apply ``next`` method on it.

However, we can convert it into ``iterator`` after applying ``iter`` method on it.

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.. code-block:: default


    prov_iterator = iter(provinces)
    next(prov_iterator)





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 .. code-block:: none


    'Sindh'



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but what is ``next`` good for?

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.. code-block:: default


    # uncomment following 2 lines

    #for i in range(len(provinces)):
    #    print(next(prov_iterator))








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we get ``StopIteration`` exception after we have iterated through all the items of list

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Let's create the iterator again and run this code.

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.. code-block:: default


    prov_iterator = iter(provinces)
    for i in range(len(provinces)):
        print(next(prov_iterator))





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    Sindh
    Balochistan
    Janubi Pubjab
    Hazara
    Pakhtunkhwa




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Now try `next` on `prov_iterator` once again

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.. code-block:: default


    # uncomment following line
    # next(prov_iterator)








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So we can get next item from an iterator until it has returned all items
from the iterable.

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.. code-block:: default


    print(type(prov_iterator))





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    <class 'list_iterator'>




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Let's create an iterator from `tuple`

.. GENERATED FROM PYTHON SOURCE LINES 80-85

.. code-block:: default


    provinces = ("Sindh", "Balochistan", "Janubi Pubjab", "Hazara", "Pakhtunkhwa")
    prov_iterator = iter(provinces)
    print(type(prov_iterator))





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    <class 'tuple_iterator'>




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Do we need a better way to check if an object is iterator or not?

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``isinstance``
----------------
can also be used if some object is of a particular type or not.

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.. code-block:: default


    isinstance(2.5, float)





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    True



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    isinstance(2.5, str)





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    False



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.. code-block:: default


    from collections.abc import Iterator, Iterable
    isinstance(prov_iterator, Iterable)





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    True



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.. code-block:: default


    isinstance(prov_iterator, Iterator)





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    True



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.. code-block:: default


    sequences = {
                 list: ["Islamabad", "Tehran", "Istambul"],
                 str: "Islamabad",
                 tuple: ("Islamabad", "Tehran", "Istambul"),
                 dict: {"Islamabad": '', "Tehran": '', "Istambul": ''},
                 int: 2,
                 float: 2.5
    }

    for _type, obj in sequences.items():
        print(isinstance(obj, _type))





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 .. code-block:: none

    True
    True
    True
    True
    True
    True




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.. code-block:: default


    for obj in sequences.values():
        print(isinstance(obj, Iterable), isinstance(obj, Iterator))





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 .. code-block:: none

    True False
    True False
    True False
    True False
    False False
    False False




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.. code-block:: default


    for obj in sequences.values():
        if isinstance(obj, Iterable):
            obj = iter(obj)
            print(isinstance(obj, Iterable), isinstance(obj, Iterator))
        else:
            print("{} can not be converted into iterator because it is not iterable".format(obj))





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    True True
    True True
    True True
    True True
    2 can not be converted into iterator because it is not iterable
    2.5 can not be converted into iterator because it is not iterable




.. GENERATED FROM PYTHON SOURCE LINES 139-149

The key takeaways from above two cells are following:

* So ``list``, ``str``, ``tuple`` and ``dict`` are iterables as such and not iterators.
However they can be converted into ``iterators`` after applying `next` method on it.

* Every ``iterator`` is also and ``iterable`` but all iterables are not iterators.

* In order to be able to create ``iterator`` from an object, it must be ``iterable``
first. Floats and integers are not iterable so can not be converted into iterators as well.


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As an ``iterator`` is also ``iterable`` so it can also be used on right side of ``for`` loop.

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.. code-block:: default


    for prov in prov_iterator:
        print(prov)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    Sindh
    Balochistan
    Janubi Pubjab
    Hazara
    Pakhtunkhwa




.. GENERATED FROM PYTHON SOURCE LINES 159-164

If you run the above cell again, it will not print anything because the iterator
is already exhausted.

Not all methods that can be applied on an ``iterable`` can also be applied on
``iterator``. For example the ``len`` method can not be applied on `iterator`.

.. GENERATED FROM PYTHON SOURCE LINES 166-170

.. code-block:: default


    # uncomment following line
    # len(prov_iterator)








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we can however find the number of items in an iterator after converting it into list

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.. code-block:: default


    prov_iterator = iter(provinces)
    len(list(prov_iterator))





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 .. code-block:: none


    5



.. GENERATED FROM PYTHON SOURCE LINES 179-182

The methods ``next`` and ``iter`` for iterator and iterable actually come from
``__next__`` and ``__iter__`` methods which reside inside corresponding class.
Since OOP concepts are not discussed in this course, so these methods are also not discussed.


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