The Hidden Power of Stateful Iterators in Python

Take control of iteration with objects that remember their state

Photo by Evgeniy Smersh on Unsplash

Most basic iterators in Python are stateless — they simply return values from a sequence without remembering anything. But by creating a custom iterator class, you can define stateful iterators that keep track of iteration state internally. This unlocks new capabilities for complex iteration patterns.

Defining Iterator Classes

To create a stateful iterator, define a class with iter() and next() methods. The iter() method should return self, and next() should handle retrieving and updating the state.

class CountUp:
    def __init__(self, start=0, step=1):
        self.current = start
        self.step = step
    
    def __iter__(self):
        return self
    
    def __next__(self):
        result = self.current
        self.current += self.step
        return result

counter = CountUp(3, 2)
print(next(counter)) # 3
print(next(counter)) # 5
print(next(counter)) # 7

This CountUp iterator tracks its current value as internal state, updating it each time next() is called.

Preserving Iteration State