How to Create Custom Iterator in Python: Simple Guide

To create a custom iterator in Python, define a class with __iter__ method returning the iterator object and a __next__ method that returns the next item or raises StopIteration when done. This lets you control how your object is looped over in for loops or other iteration contexts.

Syntax

To create a custom iterator, define a class with these two methods:

• __iter__(self): Returns the iterator object itself. Usually, this is self.
• __next__(self): Returns the next item in the sequence. Raises StopIteration when no more items are left.

python

class MyIterator:
    def __iter__(self):
        # Return the iterator object
        return self

    def __next__(self):
        # Return the next item or raise StopIteration
        pass

Example

This example creates an iterator that returns numbers from 1 to 5. It shows how __iter__ and __next__ work together to produce values one by one.

python

class CountToFive:
    def __init__(self):
        self.current = 1

    def __iter__(self):
        return self

    def __next__(self):
        if self.current <= 5:
            number = self.current
            self.current += 1
            return number
        else:
            raise StopIteration

# Using the custom iterator
for num in CountToFive():
    print(num)

Output

1
2
3
4
5

Common Pitfalls

Common mistakes when creating custom iterators include:

• Not raising StopIteration when the sequence ends, causing infinite loops.
• Forgetting to return self in __iter__.
• Modifying the iteration state incorrectly, leading to skipped or repeated items.

Here is a wrong and right way example:

python

# Wrong: __iter__ does not return self
class WrongIterator:
    def __iter__(self):
        return []  # Should return self
    def __next__(self):
        raise StopIteration

# Right:
class RightIterator:
    def __iter__(self):
        return self
    def __next__(self):
        raise StopIteration

Quick Reference

Method	Purpose
__iter__(self)	Returns the iterator object (usually self)
__next__(self)	Returns the next item or raises StopIteration
StopIteration	Exception to signal no more items

Key Takeaways

Define __iter__ to return the iterator object itself.

Define __next__ to return the next item or raise StopIteration when done.

Always raise StopIteration to end iteration cleanly.

Use your custom iterator in for loops or any iteration context.

Test your iterator to avoid infinite loops or missing items.