Which protocol makes an object usable in a for loop?

Short answer

The object must define __iter__ returning an iterator, and the iterator defines __next__ that returns the next item and raises StopIteration at the end (and defines __iter__ returning self).

Explanation

A for loop calls iter() once and then next() repeatedly until StopIteration. The easiest way to implement the protocol is to make __iter__ a generator function. Keep the collection separate from the iteration state: if you store the iteration position inside the object itself, you cannot iterate it twice independently or in a nested way.

Code example

class Countdown:
    def __init__(self, start: int):
        self.start = start

    def __iter__(self):  # Each call starts an independent iteration
        n = self.start
        while n > 0:
            yield n
            n -= 1

print(list(Countdown(3)))  # [3, 2, 1]

Common mistakes

• Storing the iteration state in the object itself, so it can be iterated only once.
• Returning None from __iter__ instead of an iterator.

Follow-up questions

• What is the difference between an iterable and an iterator?

Sources

• Python documentation — Iterator types