Understanding the Iterator Protocol in Python
Tags: python, beginners, tutorial, books
Published: 2021-11-01 11:22:48

*This is an excerpt from our  [free Python course](https://primerlabs.io/books/python-i/)  on  [Primer](https://primerlabs.io).* 

Iterables &  Iterators are at the core of Python. Let's learn what are they and how to understand them better. 

Some definitions first:

**Iterables**: An object capable of returning its member one at a time is called an iter-able.

To get a member from an iterable, one at a time, Python provides a built-in function `iter()`. When we provide an iterable as an argument to the `iter()` function ( which in returns call the `__iter__()` **dunder method** on the object ), it returns something called **iterator**.

Let's check out in code:
```python
>>> guests = {'Luffy', 'Zorro', 'Sanji'}
>>> iter(guests)
<set_iterator object at 0x7f7983d1dab0>
```

The above code listing provides the guests set to the iter() returns a set_iterator object. But what's an iterator?

- An iterator is an object representing a stream of data.
When you repeatedly call the built-in `next()` on the iterator object, it returns the stream's next items.
- When no more items are available in the stream of data, the StopIteration exception is raised instead.

To understand this, let's continue our code from above.

```python
>>> guests = {'Luffy', 'Zorro', 'Sanji'}
>>> guest_iterator = iter(guests)
>>> next(guest_iterator)
'Zorro'                        # Sets are unordered
>>> next(guest_iterator)
'Luffy'
>>> next(guest_iterator)
'Sanji'
>>> next(guest_iterator)    # No more items left in the set
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration
```

In the above code listing, we can see how the iterator returned from guests can provide one object at a time using built-in `iter()` and `next()` functions.

For an object to be iterable, it needs to have the dunder method, `__iter__()` defined. It's the `__iter__()` method, which returns an iterator from an iterable.

he built-in objects such as `sequences`, `sets`, and `dictionaries` have the dunder method `__iter__()` defined, which lets us use in `for` loop.

Now that we have gained a bit of insight into how Python implements iterations let's try to write a `while` loop to iterate over a set of objects.

```python
>>> guests = {'Luffy', 'Zorro', 'Sanji'}
>>> guest_iterator = iter(guests)    # Same as guests.__iter__()
>>> while True:
...     try:
    		# Same as guest_iterator.__next__()
...         guest = next(guest_iterator)
...         print(guest)
...     except StopIteration as e:
...         break
Zorro
Luffy
Sanji
```

The `while` loop we wrote is pretty close to what happens under the hood when we iterate on an iterable using a `for` loop. Although, when using iterables with the for loop, we don't need to call the `iter()` function or handle the `StopIteration` error as the for statement does that automatically for us.


We can describe the iterator protocol as the following:

- First, obtain the iterator of the object using the `iter()` function or the dunder method `<iterable>.__iter__()`.
Call the built-in function `next()` or the dunder method `<iterator>.__next__()` on the iterator object.
- Run the code block inside the `for` block
- Repeat the next invocation until the iterator raises `StopIteration`


![Iterators,+Generators+and+Comprehensions+-+Iterator+Protocol.svg](https://assets.primerlabs.io/CourseAssets/Python1/Chapter6/Iterators%2C+Generators+and+Comprehensions+-+Iterator+Protocol.png)

Let's redefine our redefine the terms iterable and iterator.

**Iterable**: An object that allows the iteration is called an iter-able. The iterable is required to have an `__iter__()` method defined that returns an iterator.

**Iterator**: The iterator is required to have both an `__iter__()` method and a `__next__()` method.

So far, we can summarise everything we have learned about iterables and iterators in the table below.



![Chapter-5-Iterators-Generators-Comprehensions (1).png](https://cdn.hashnode.com/res/hashnode/image/upload/v1635765435289/CWj5KKr7E.png)

In this section, we covered the difference between iterators and iterables. If you wish to learn Python in-depth, you can visit  [Primer](https://primerlabs.io)  and check out our free course on Python. You can also follow me on [twitter](https://twitter.com/_wzard).