1.16 iterator vs iterable

Important

This lesson is still under development.

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)

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

Sindh
Balochistan
Janubi Pubjab
Hazara
Pakhtunkhwa

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

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.

prov_iterator = iter(provinces)
next(prov_iterator)

'Sindh'

but what is next good for?

# uncomment following 2 lines

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

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

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

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

Sindh
Balochistan
Janubi Pubjab
Hazara
Pakhtunkhwa

Now try next on prov_iterator once again

# uncomment following line
# next(prov_iterator)

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

print(type(prov_iterator))

<class 'list_iterator'>

Let’s create an iterator from tuple

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

<class 'tuple_iterator'>

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

isinstance

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

isinstance(2.5, float)

True

isinstance(2.5, str)

False

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

True

isinstance(prov_iterator, Iterator)

True

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))

True
True
True
True
True
True

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

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

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))

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

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.

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

for prov in prov_iterator:
    print(prov)

Sindh
Balochistan
Janubi Pubjab
Hazara
Pakhtunkhwa

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.

# uncomment following line
# len(prov_iterator)

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

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

5

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.