Source: 📖 Python Cookbook ch4.2 p114

Iterating through a custom object

Sometimes you'll create a custom object that contains an iterable attribute. You can define an __iter__ method to be able to iterate through this attribute by calling a for loop directly on the object itself, which makes for cleaner code than accessing internal attributes each time.

class MyObject:
	def __init__(self):
		self._internal_iterable = [1, 2, 3, 4]
	
	def __iter__(self)
		return iter(self._internal_iterable)
		
test = MyObject()

for num in test:
	print(num)

# 1
# 2
# 3
# 4

Each time a for loop is called, an iterator object is created by calling .__iter__() on the object in question. In the above example, we are simply instructing the .__iter__() method to return an iterator of the _internal_iterable attribute. So when a for loop is called on the overarching object, iter() is called on _internal_iterable and the resulting iterator object is returned, ready to be looped through.