Iter

Iter is commonly used to refer to the Python built-in function iter, which returns an iterator for a given iterable. There are two forms: iter(iterable) and iter(callable, sentinel). The first creates an iterator from an iterable object by calling its __iter__ method. The second creates an iterator that repeatedly calls a zero-argument function until it returns a sentinel value; when the sentinel is produced, iteration ends.

An iterator implements the iteration protocol by providing __iter__ and __next__ methods. __iter__ returns the iterator itself, and __next__ returns the next value or raises StopIteration when there are no more items. Iterators are stateful, advancing through a sequence as next() is called. The underlying object is typically described as an iterable if it can produce an iterator via __iter__ (or support indexed access via __getitem__ in older designs).

Common iterables include lists, tuples, strings, and dictionaries (which yield keys by default). You obtain an iterator with iter(obj) and advance it with next(it). Most high-level constructs, such as for loops, use iterators implicitly. You can also create a list from an iterable with list(obj) or from an iterator with list(it). When exhausted, subsequent calls to next(it) raise StopIteration, and a for loop ends automatically.

Notes: The distinction between iterable and iterator is important. An iterable can produce a new iterator each time, while an iterator is consumable and cannot be reset unless you obtain a new one from the iterable. Some objects are both iterable and iterators, such as generator objects created by generator expressions.