How to Implement __iter__ and __next__ in Python: Simple Guide

To implement __iter__ and __next__ in Python, define __iter__ to return the iterator object itself and __next__ to return the next item or raise StopIteration when done. This makes your object usable in loops like for.

Syntax

To make an object iterable, define 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 self

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

Example

This example shows a simple iterator that counts from 1 to 3. It demonstrates how __iter__ returns the object and __next__ returns the next number or stops iteration.

python

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

    def __iter__(self):
        return self

    def __next__(self):
        if self.current <= 3:
            number = self.current
            self.current += 1
            return number
        else:
            raise StopIteration

counter = CountToThree()
for num in counter:
    print(num)

Output

1
2
3

Common Pitfalls

Common mistakes include:

• Not returning self in __iter__, which breaks the iterator protocol.
• Forgetting to raise StopIteration in __next__, causing infinite loops.
• Modifying the iterable inside __next__ incorrectly, leading to wrong results.

python

class WrongIterator:
    def __iter__(self):
        # Wrong: returns a new object instead of self
        return []

    def __next__(self):
        # Missing StopIteration
        return 1

# Correct version
class RightIterator:
    def __init__(self):
        self.count = 0

    def __iter__(self):
        return self

    def __next__(self):
        if self.count < 3:
            self.count += 1
            return self.count
        else:
            raise StopIteration

Quick Reference

Remember these key points when implementing iterators:

• __iter__ must return the iterator object itself.
• __next__ must return the next item or raise StopIteration.
• Use iterators to enable for loops and other iterable contexts.

Key Takeaways

Implement __iter__ to return self to make your object an iterator.

Implement __next__ to return the next item and raise StopIteration when done.

Always raise StopIteration to signal the end of iteration.

Iterators let your objects work with for loops and other iterable contexts.

Avoid returning new objects in __iter__; return self instead.