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Custom Iterators in Python

Advanced Iteration

Iterator Protocol Deep Dive

Every for-loop depends on the iterator protocol. Build your own iterable objects and unlock streaming patterns.

Implement __iter__/__next__Expose iterables from classes

Protocol​

class Countdown:
    def __init__(self, start: int):
        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
  • __iter__ returns the iterator (often self).
  • __next__ raises StopIteration to end the loop.

Yield from objects​

Even if you don’t implement __next__, returning an iterator from __iter__ makes your object iterable:

class LessonPlan:
    def __init__(self, topics):
        self.topics = topics

    def __iter__(self):
        return iter(self.topics)

Next up in your learning path

Generators

Use `yield` for concise iterator definitions.

Yield

itertools

Compose iterators using standard utilities.

Companion

Context managers

Combine iteration with managed resources.

Abstraction