We have seen that the methods __iter__() and __next__() must be implemented for an object to be an iterator object. The implementation of these methods is known as the iterator protocol.

If we desire to create our own custom iterator class, we must implement the iterator protocol, meaning we need to have a class that defines at minimum the __iter__() and __next__() methods.

To look at a scenario where we might require our own custom iterator, imagine we are receiving a shipment of new fish that we can now sell in our pet store. We don’t have any classes to manage our fish inventory, so we need to create a custom class to do so. If we wanted to track the available fish inventory, our custom class initializer may look something like this:

class FishInventory:
  def __init__(self, fishList):
      self.available_fish = fishList

By default, custom classes are not iterable. We can’t just go around plugging our custom classes into for loops and expecting any results! This presents a problem if the class we are working with needs the ability to iterate.

When we create a FishInventory class object, we want to iterate over all the fish available within self.available_fish. If we attempt to directly iterate over our custom FishInventory class object, we will receive an error because we have not yet implemented the iterator protocol for this custom class. To make the FishInventory class iterable, we can simply define __iter__() and __next__() methods.

Defining these methods is discussed in the next exercise, but first, let’s see what happens if we attempt to iterate our FishInventory class object directly.