Now that we have our iterator object, how does our for loop know which value to retrieve on each iteration?

Well, in addition to implementing the __iter__() method, iterator objects also implement a method called __next__(). The __next__() method retrieves the iterator’s next value. Let’s take a look using our SKU iterable for our shop:

sku_list = [7046538, 8289407, 9056375, 2308597]
sku_iterator = iter(sku_list)
next_sku = sku_iterator.__next__()
print(next_sku)

Running this code would produce the following result for next_sku:

7046538

Similarly to __iter__() and iter(), there is a Python built-in function called next() that we can use in place of calling the __next__() method. Calling next() simply calls the iterator object’s __next__() method. Here is the same script but using next():

sku_list = [7046538, 8289407, 9056375, 2308597]
sku_iterator = iter(sku_list)
next_sku = next(sku_iterator)
print(next_sku)

Running this code is exactly the same as running the code above using the __next__() method and produces the same next_sku result:

7046538

But how does the iterator object know when to stop retrieving values? Does it keep calling __next__() forever? Well, luckily __next__() method will raise an exception called StopIteration when all items have been iterated through.

If we call __next__() a total of 5 times, one more than the total number of SKUs in our list, we will see the StopIteration exception raise on the last __next__() call:

sku_list = [7046538, 8289407, 9056375, 2308597]
sku_iterator = iter(sku_list)
for i in range(5):
  next_sku = sku_iterator.__next__()
  print(next_sku)

Running this code will produce the following output:

7046538
8289407
9056375
2308597

Followed by:

Traceback (most recent call last):
  File "main.py", line 24, in <module>
    next_sku = sku_iterator.__next__()
StopIteration

In summary, we can finally see why we needed to create the iterator object in the previous exercise. Creating it, allows us to utilize next() or __next__ to work with the stream of data one piece at a time.

Now, let’s practice getting the hang of retrieving individual iterator object values!