Python program to build a simple Iterator class

Pre-requisites

1. Python Iterators - iter() and next()

How do iterators work?

Iterator objects in Python programming confirm to the iterator protocol. They provide two methods: __iter__() and __next__() to iterate over all the elements of the iterator.

• __iter__ is called at the start of a loop. It returns the iterator object.
• __next__ method returns the next value in the iterator and is implicitly called at each loop increment. When there are no more value to return, it raises a StopIteration example.

For more information on Python iterators, check out the pre-requisite article mentioned above.

Let’s build a simple Counter class to understand the concept.

Code

class Counter:
    def __init__(self, low, high):
        self.current = low - 1
        self.high = high

    def __iter__(self):
        return self

    def __next__(self):
        self.current += 1
        if self.current < self.high:
            return self.current
        raise StopIteration

for elem in Counter(1, 10):
    print(elem)

Output

1
2
3
4
5
6
7
8
9

The above is a basic iterator class built with Python Object Oriented Programming.

This is where generators come to rescue.

Code

def counter(low, high):
    current = low
    while current < high:
        yield current
        current += 1

for elem in Counter(1, 10):
    print(elem)

Output

1
2
3
4
5
6
7
8
9

As you can see, the printed output will be the same.