Iterators and iterables

An iterable is any object that can return an iterator, by implementing dunder iter. An iterator is an object that produces values one at a time, by implementing both dunder iter and dunder next. Lists and dicts are iterables but not iterators. Calling iter on an iterable returns an iterator.

A for loop calls iter on the iterable to get an iterator, then repeatedly calls next on the iterator. Each call returns the next value, which is bound to the loop variable. When the iterator is exhausted, it raises StopIteration, and the loop exits cleanly without propagating the exception.

StopIteration is the exception raised by an iterator's next method to signal that there are no more values. The for loop catches it automatically. Manually iterating with next can let you handle StopIteration explicitly. Inside generators, returning from the function raises StopIteration implicitly.

Implement dunder iter on the class. The method should return an iterator, which can be the object itself if it also implements dunder next, or a separate iterator object. For most cases, returning a generator from dunder iter is the cleanest approach.

The iter function returns an iterator from an iterable. It calls the iterable's dunder iter method. There is also a two-argument form: iter of a callable and a sentinel returns an iterator that calls the callable until it returns the sentinel. This is useful for iterating over file reads or network responses. ---