__iter__ and __next__

Question

Accepted Answer

Any object that implements both methods is an iterator. Python's for loop calls iter(obj) first to get the iterator, then repeatedly calls next() until StopIteration is raised. An iterable (like a list) implements __iter__ to return a fresh iterator, but the iterator itself also implements __iter__ returning self - that's what lets you use an iterator directly in a for loop. Generators implement both automatically, which is why they work seamlessly in loops.