Python Tutorial 9.4 - Iterators & Iterables

Welcome to the “Iterators & Iterables” tutorial!

Overview

• Iterable: An object you can loop over (supports __iter__()).
• Iterator: The object returned by iter() (supports __next__()).

Example

nums = [1, 2, 3]
it = iter(nums)
print(next(it))  # 1
print(next(it))  # 2

Custom Iterator

class CountDown:
    def __init__(self, start):
        self.current = start

    def __iter__(self):
        return self

    def __next__(self):
        if self.current <= 0:
            raise StopIteration
        value = self.current
        self.current -= 1
        return value

for num in CountDown(3):
    print(num)  # 3, 2, 1

Key Takeaways

• Use iter() and next() to manually control iteration.
• Implement __iter__() and __next__() for custom iterators.
• StopIteration signals the end of an iterator.

Happy coding!