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.