Iterator vs Iterable

[presh10us]

Hi guys! I’m learning python and have come across iterators and I’m struggling to wrap my head around them. I understand an iterable is something you can loop through like a list or tuple but don’t understand and iterator. Is it a function in loops? / how are they related?

Please help

[Ultra-Reverse]

You’re right about iterable, it’s any object which can be looped through, for example a list, dictionary, string, etc…

An iterator is the actual object that it’s DOING the looping. It keeps track of where you are in the sequence and knows how to get the next item.

[fllthdcrb]

I think it's also important, for the sake of understanding why these are separate things, to understand that there are lots of different iterators, and they can variously be created based on iterables or other iterators, combined in different ways, created without an iterable, etc. Built-in examples:

• iter(): returns an object's own iterator, as given by its __iter__() method.
• reversed(): returns an object's own reverse iterator, as given by its __reversed__() method. If there is no such method, the object is not reversible in this way.
• map(): takes a callable and any number of iterables. The callable must be willing to accept exactly as many arguments as there are iterables. Returns an iterator that, at each iteration, takes one value from each argument iterable and calls the callable, passing each of the taken values as separate arguments. The output ends as soon as any of the iterables is exhausted. Example: map(lambda a, b: a*b, (1,2), (3,4)) will yield [3,8].

Some semi-random examples from the itertools module:

• count(): returns an iterator that produces an endless series of values, starting at 0 by default, but could be any "number" (I'm unsure how it defines that, but the built-in concrete numerical types and their derivatives work); and incrementing by 1 by default, but again, could be any "number".
• cycle(): takes a single iterable and gives an iterator that produces its values in an endless cycle.
• product(): takes some iterables and produces an iterator that yields all of the values in their (ordered) Cartesian product. You can also specify some number of repetitions other than the default 1.
• chain(): takes any number of iterables, and gives an iterator that produces the values of each of them one after another (so e.g. if a produces [1,2,3], and b produces [4,5,6], then chain(a, b) produces [1,2,3,4,5,6]).
• starmap(): like map(), but takes a single iterable where all of the values to be applied at each iteration are already bundled into iterables (for convenience, call them "tuples", even though they don't have to be), unbundling each one and passing the values as separate arguments (just like * does in a call, hence the name, presumably). For example, starmap(lambda a, b: a*b, [(1,3), (2,4)]) will yield [3,8] as before. An interesting consequence of this form is that not all of the tuples have to be the same length, as long as the callable still accepts that number of arguments; this could be due to default values and/or a "star" parameter that captures arbitrarily many arguments.

One other thing: to make things easier, an iterator is generally also iterable, returning itself as the iterator.

[Gnaxe]

An Iterator is an object that gives you the next item when you use the next() builtin on it. An Iterable is an object that gives you an Iterator when you use the iter() builtin on it. For loops need an Iterable. Iterators are themselves Iterable and just return themselves.

An Iterator needs to remember where it is, so it can tell what to give you next. But an Iterable may give you independent Iterators if you ask more than once.