Python __iter__() and __next__() | Converting an object into an iterator

In many instances, we get a need to access an object like an iterator. One way is to form a generator loop but that extends the task and time taken by the programmer. Python eases this task by providing a built-in method __iter__() for this task. In this article, we will see about Python __iter__() and __next__().

Python __iter__()

The __iter__() function in Python returns an iterator for the given object (array, set, tuple, etc., or custom objects). It creates an object that can be accessed one element at a time using __next__() in Python, which generally comes in handy when dealing with loops.

Syntax

iter(object)
iter(callable, sentinel)

• Object: The object whose iterator has to be created. It can be a collection object like a list or tuple or a user-defined object (using OOPS).
• Callable, Sentinel: Callable represents a callable object, and sentinel is the value at which the iteration is needed to be terminated, sentinel value represents the end of the sequence being iterated.

Exception

If we call the iterator after all the elements have 
been iterated, then StopIterationError is raised.

The __iter__() in Python returns an iterator object that goes through each element of the given object. The next element can be accessed through __next__() in Python. In the case of the callable object and sentinel value, the iteration is done until the value is found or the end of elements is reached. In any case, the original object is not modified.

In this example, we are using the iter and next function to iterate through the list and print the next element in the next line.

Python3

# Python code demonstrating
# basic use of iter()
listA = ['a','e','i','o','u']

iter_listA = iter(listA)

try:
    print( next(iter_listA))
    print( next(iter_listA))
    print( next(iter_listA))
    print( next(iter_listA))
    print( next(iter_listA))
    print( next(iter_listA)) #StopIteration error
except:
    pass

Output

a
e
i
o
u

Python __next__()

Python __next__() is responsible for returning the next element of the iteration. If there are no more elements then it raises the StopIteration exception. It is part of the iterable and iterator interface, which allows us to create custom iterable objects, such as generators, and control how elements are retrieved one at a time from those iterables.

In this example, we are using __next__() function in Python to iterate and print next element inside the list.

Python3

# Python code demonstrating
# basic use of iter()
lst = [11, 22, 33, 44, 55]

iter_lst = iter(lst)
while True:
    try:
        print(iter_lst.__next__())
    except:
        break

Output

11
22
33
44
55

In this example, we are using __next__() in Python to show the exception that is thrown if next element is not present.

Python3

# Python code demonstrating
# basic use of iter()

listB = ['Cat', 'Bat', 'Sat', 'Mat']


iter_listB = listB.__iter__()

try:
    print(iter_listB.__next__())
    print(iter_listB.__next__())
    print(iter_listB.__next__())
    print(iter_listB.__next__())
    print(iter_listB.__next__())
except:
    print(" \nThrowing 'StopIterationError'",
                     "I cannot count more.")

Output

Cat
Bat
Sat
Mat
 
Throwing 'StopIterationError' I cannot count more.

User-defined objects (using OOPS)

In this example, we are using user defined objects along with defining __iter__() and __next__() functions to show the use of iter() using OOPS in Python.

Python3

class Counter:
    def __init__(self, start, end):
        self.num = start
        self.end = end

    def __iter__(self):
        return self

    def __next__(self):
        if self.num > self.end:
            raise StopIteration
        else:
            self.num += 1
            return self.num - 1

# Driver code
if __name__ == '__main__' :
    a, b = 2, 5
    c1 = Counter(a, b)
    c2 = Counter(a, b)

    # Way 1-to print the range without iter()
    print ("Print the range without iter()")

    for i in c1:
        print ("Eating more Pizzas, counting ", i, end ="\n")

    print ("\nPrint the range using iter()\n")

    # Way 2- using iter()
    obj = iter(c2)
    try:
        while True: # Print till error raised
            print ("Eating more Pizzas, counting ", next(obj))
    except:
        # when StopIteration raised, Print custom message
        print ("\nDead on overfood, GAME OVER")

Output

Print the range without iter()
Eating more Pizzas, counting  2
Eating more Pizzas, counting  3
Eating more Pizzas, counting  4
Eating more Pizzas, counting  5

Print the range using iter()

Eating mor...