Learn

Let’s return to our for loop from before:

for food_brand in dog_foods:
    print(food_brand + " has " + str(dog_foods[food_brand]) + " bags")

Under the hood, the first step that the for loop has to do is to convert our dictionary (the iterable) of dog_foods to an iterator object. An iterator object is a special object that represents a stream of data that we can operate on. To accomplish this, it uses a built-in function called iter():

dog_food_iterator = iter(dog_foods)

We can see the new object by printing it:

print(dog_food_iterator)

This would output our new iterator object:

<dict_keyiterator object at 0x....> 

# Note: The memory address is omitted since it varies on the system you run the script on.

Here is a visual representation, using the graphic we saw earlier: Iter method

To go behind the scenes even further, iter(dog_foods) is actually calling a method defined within the iterable called __iter__(). All iterables have this __iter__() method defined. We can even use the Python built-in function dir() to show that our dog_foods dictionary (iterable) has a defined method called __iter__().

print(dir(dog_foods))

If we examined the output (shortened for brevity), we can spot the __iter__ property

['__class__', '__contains__', '__delattr__', '__delitem__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__iter__'...<----- iter method

In summary, the __iter__() method simply returns the iterator object that allows us to iterate over the iterable. Calling dog_foods.__iter__() will retrieve the same iterator object as calling iter(dog_foods). This means that the built-in function iter() and the iterable’s method __iter__() can be used interchangeably. While the object itself might not seem super useful just yet, we’ll see how to manipulate the stream of data inside of it in the next exercises.

Now that we have taken a peek under the hood, let’s practice creating our own iterator objects from iterables using iter() and __iter__().

Instructions

Suppose we have a list of SKUs (stock-keeping units) for products in our pet shop. Let’s examine the internal methods of the iterable sku_list.

Use dir() on sku_list and print out the result. Can you spot __iter__ in the list of methods that are printed?

Let’s access the internal __iter__() method from sku_list to create our iterator object.

Create a variable called sku_iterator_object_one that is equal to calling .__iter__() on sku_list.

Lastly, print sku_iterator_object_one!

Finally, let’s use the alternative iter() function to create an iterator object from sku_list

Create a variable called sku_iterator_object_two that is equal to calling iter() on sku_list.

Lastly, print sku_iterator_object_two!

Observe that both methods will be able to retrieve an iterator object but it’s always helpful to know that iter() uses __iter__ under the hood.

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