All 15 Iterators & the Iterator Protocol questions
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What is the difference between an iterable and an iterator in Python? What exception signals the end of iteration, and how does a for-loop use it internally? What does the built-in next() function's optional default argument do? How do you manually exhaust and inspect an iterator without a for-loop? How does Python's iter() function work with a sentinel value as a second argument? How does the itertools module complement the iterator protocol? How does Python know whether to treat a for-in target as iterable, and what error occurs if it isn't? What is the relationship between generators and the iterator protocol? What is the difference between iter(obj) and using obj directly in a for-loop, in terms of what Python calls internally? What is the difference between __iter__ returning self versus returning a separate iterator object?
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Python › Iterators & the Iterator Protocol

How do you implement a custom iterator by defining __iter__ and __next__?

Intermediate

Answer

A custom iterator class defines __next__() to return the next value or raise StopIteration when exhausted, and __iter__() to return self so the object works directly in for-loops and with the built-in iter() function.
class Countdown:
    def __init__(self, start):
        self.current = start
    def __iter__(self):
        return self
    def __next__(self):
        if self.current <= 0:
            raise StopIteration
        self.current -= 1
        return self.current + 1

for n in Countdown(3):
    print(n)  # 3 2 1
Real-world example A custom paginated API client implements __next__ to fetch and yield the next page of results lazily as the caller iterates.

Common follow-ups: Why must __iter__ return self on an iterator?;How does this differ from a generator-based approach?

Iterators & the Iterator Protocol;Comprehensions & Generators
All 15 Iterators & the Iterator Protocol questions