How would you make a custom class usable in a for loop?
Implement iter returning an iterator, or generate with a generator method
The for loop calls iter(obj) once and then next() until StopIteration. So you need either iter that returns an iterator, or the legacy getitem protocol that yields values by integer index until IndexError. The cleanest modern approach is to make iter a generator function: writing def iter(self): for item in self._data: yield item turns iter into a generator function, so each call returns a fresh generator. That gives you re-iterability for free and keeps the code short. If your object is itself a one-shot stream, you can implement iter returning self plus next instead.
- 1
Prefer a generator-based iter for containers because it is concise and re-iterable.
- 2
Implement next plus iter returning self only when the object is inherently a one-shot iterator, such as a socket reader.
- 3
getitem with integer indexing still works for the for loop but is legacy; avoid it for new code except for compatibility.
- 4
Trade-off: generator iter is easier but hides the iteration logic; an explicit iterator class is more verbose but easier to instrument and reset.
- 5
Common mistake: implementing next without iter and then finding that for loops raise TypeError: object is not iterable.
- 6
Version note: iter/next is the modern protocol (PEP 234). getitem iteration is a backward-compatibility fallback and is not recommended for new code.
Scenario Questions
0-2 years experience
- 1You implement __next__ but not __iter__ and for loops fail. Why?
- 2What two methods does the iterator protocol require?
2-5 years experience
- 1You need a class that can be iterated twice independently. How do you implement it?
- 2Your class wraps a file-like reader and should be iterated only once. Which pattern applies?
5-8 years experience
- 1You need iteration that supports early exit and cleanup of an external resource. How do you structure __iter__?
- 2You are building a paginated client where each page is fetched lazily. How do __iter__ and pagination interact?
8+ years experience
- 1Design a class that supports iteration, indexing, slicing, membership testing, and reversed() with minimal dunder implementation.
- 2Explain how generator-based __iter__ interacts with context managers and close() when the consumer abandons iteration early.
Follow-up Questions
- When would returning self from __iter__ be correct rather than a bug?
- How does the legacy __getitem__ protocol decide when to stop iterating?