Iterable & Iterator Protocol (__iter__ & __next__)

1Concept

An Iterable is an object providing an `__iter__()` method that returns an Iterator. An Iterator implements `__next__()` to return subsequent elements and raises `StopIteration` when exhausted.

2Architecture Diagram

[ Iterable Object ] ---> .__iter__() ---> [ Iterator Object ] ---> .__next__() ---> Item 1
                                                                           ---> .__next__() ---> Item 2
                                                                           ---> .__next__() ---> StopIteration

3Code Example

class FibonacciIterator:
    def __init__(self, limit: int):
        self.limit = limit
        self.count = 0
        self.a, self.b = 0, 1

    def __iter__(self):
        return self

    def __next__(self):
        if self.count >= self.limit:
            raise StopIteration
        val = self.a
        self.a, self.b = self.b, self.a + self.b
        self.count += 1
        return val

fib = FibonacciIterator(6)
print(f"Fibonacci Sequence: {list(fib)}")

4Expected Output

Fibonacci Sequence: [0, 1, 1, 2, 3, 5]

5Key Takeaways

• ✓An iterator is exhausted after a single complete traversal.
• ✓The `for` loop catches `StopIteration` automatically behind the scenes.
• ✓Iterators consume O(1) memory regardless of sequence length.