In this article, I will take you through the Concepts of Iterator in Python Explained with Best Examples. Iterator is very handy in iterating over the list, tuples, dict, set etc. in python hence it plays an important role in Python Scripting. Iteration is fundamental to data processing - programs mostly apply computations to data series, from pixels to nucleotides. If the data does not fit in memory, we need to fetch the items lazily - one at a time and on demand. That’s what an iterator does. This article shows how the Iterator pattern is built into the Python language so you never need to code it by hand.

Every collection in Python is iterable, and iterators are used internally to support:-

• for loops
• Collection types construction and extension
• Looping over text files line by line
• List, dict, and set comprehensions
• Tuple unpacking
• Unpacking actual parameters with * in function calls

Why Sequences are Iterable

Whenever the interpreter needs to iterate over an object x, it automatically calls iter(x).

The iter built-in function:

• Checks whether the object implements __iter__, and calls that to obtain an iterator.
• If __iter__ is not implemented, but __getitem__ is implemented, Python creates an iterator that attempts to fetch items in order, starting from index 0 (zero).
• If that fails, Python raises TypeError, usually saying “C object is not iterable,” where C is the class of the target object.

Iterable vs Iterator

Any object from which the iter built-in function can obtain an iterator. Objects implementing an __iter__ method returning an iterator are iterable. Sequences are always iterable; as are objects implementing a __getitem__ method that takes 0-based indexes. It’s important to be clear about the relationship between iterables and iterators: Python obtains iterators from iterables. Check more about Iterator in Python on Fluent Python.

Using Iterator in Python Explained with 3 Best Examples

Also Read: Concept of Data Encapsulation in Python Explained with Best Examples

In python, iterators are objects that can be iterated upon. lists, dictionary, tuples and sets in python are all iterable objects.

All iterator objects uses two exclusive functions. They are:-

iter() -> which is used for initializing iterator object. It internally calls the method __iter__()

next() -> which is used for iteration i.e iterate all the items of iterator one by one. In python3, we can use __next__() instead.

Above two methods are collectively called as iterator protocol.

Example 1: Python in-built iterator

mylist= [3,2,4,'Hi']
mystring= 'cyberithub'
iterobject1= iter(mystring)
iterobject= iter(mylist)
print(next(iterobject))  #returns 1 item at a time
#print(iterobject.__next__())  #  use iterobject.__next__() in python3
for i in iterobject:     #iterate complete list right from the next element
    print(i)
for j in iterobject1:
    print(j)

Program Output

[root@localhost ~]# python example.py
3
2
4
Hi
c
y
b
e
r
i
t
h
u
b

Example 2: Using try-except block in Python

mystring= 'cyberithub'
iterobject= iter(mystring)
while True:
try:
print(next(iterobject))

for i in iterobject:
print(i)

except StopIteration:
print("This is the end of list")
break

Program Output

[root@localhost ~]# python example.py
c
y
b
e
r
i
t
h
u
b
This is the end of list

Example 3: Using Custom build iterators in Python

class Number:
def __init__(self, max):
self.max = max

def __iter__(self):
self.n = 0

return self

def __next__(self):
if self.max>= self.n:
result =self.max**2

self.max -= 1
return result
else:
raise StopIteration

ob = Number(6)
x = iter(ob)
print(next(x))
print(next(x))

for i in x:
print(i)

Program Output

[root@localhost ~]# python3.6 example.py
36
25
16
9
4
1
0

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