Creating an Iterator Class in Python

Building a class that generates iterator objects allows you to define custom sequences in a flexible way.

The key concept behind an iterator class is to define the behavior of an object that maintains its state during iteration and implements both the `__iter__()` and `__next__()` methods.

Let me give you a practical example.

Imagine you want to create an iterator that generates even numbers up to a specified maximum value.

class EvenNumbersIterator:
    def __init__(self, max_value):
        self.max_value = max_value  # Sets the maximum value for the even numbers to generate
        self.current = 0            # Initializes the counter at the first even number

    def __iter__(self):
        return self  # Returns the iterator object itself

    def __next__(self):
        if self.current > self.max_value:
            raise StopIteration  # Stops the iteration when the maximum value is exceeded
        else:
            even_number = self.current
            self.current += 2  # Increments to get the next even number
            return even_number  # Returns the current even number

The `__next__()` method must always include a termination condition, where it raises a `StopIteration` exception.

This is crucial to prevent infinite loops and to signal to a `for` loop that the iteration is complete.

Once the class is defined, you can create an instance of the iterator.

even_numbers = EvenNumbersIterator(10)

Since 'even_numbers' is an iterator object, you can retrieve individual values in sequence by using the `next()` function.

next(even_numbers)

2

next(even_numbers)

4

Each time you call 'next()', the next value in the sequence is returned.

You can extend the iterator's functionality to include other features, such as iterating in reverse, skipping specific numbers, or iterating over elements of a sequence based on a particular logic.