To iterate over a custom class we must implement the iterator protocol by defining the __iter__() and __next__() methods. In most cases the two methods can do the following:

• The __iter__() method must always return the iterator object itself. Typically, this is accomplished by returning self. It can also include some class member initializing.

• The __next__() method must either return the next value available or raise the StopIteration exception. It can also include any number of operations.

Let’s return to our custom FishInventory class:

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

We want to make the FishInventory class iterable to see all the available fish. To make it iterable, we first define the __iter__() method. We can initialize a class member within the __iter__() method called index that will help us track the current position we’re in within the self.available_fish list.

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

  def __iter__(self):
    self.index = 0
    return self

Notice that the __iter__() method returns itself since this class will be an iterator object. The __iter__() method can return other iterator objects, but typically the object itself is returned here by using return self.

Then, we define the __next__() method. Recall that we can perform operations inside this method, like incrementing class members or traversing a for loop for instance.

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

  def __iter__(self):
    self.index = 0
    return self

  def __next__(self):
    fish_status = self.available_fish[self.index] + " is available!"
    self.index += 1
    return fish_status

We return the next available fish status within a string value and increment our class member index by 1.

Iterating over this class object will eventually error out since we fail to do any checking of our index value against the length of the self.available_fish list. We can avoid this and cleanly stop the iterator by raising the StopIteration exception in our __next__() method. Here, we’ll modify our __next__() method to raise StopIteration if index exceeds the length of available_fish.

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

  def __iter__(self):
    self.index = 0
    return self

  def __next__(self):
    if self.index < len(self.available_fish):
      fish_status = self.available_fish[self.index] + " is available!"
      self.index += 1
      return fish_status
    else:
      raise StopIteration

Let’s practice implementing the iterator protocol in a custom class!