All 15 Iterators & the Iterator Protocol questions
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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 do you implement a custom iterator by defining __iter__ and __next__? 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? How would you implement a lazy, chainable iterator pipeline (like filter-then-map) from scratch?
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How does Python's iter() function work with a sentinel value as a second argument?

Intermediate

Answer

iter(callable, sentinel) repeatedly calls callable with no arguments and yields each result until the result equals sentinel, at which point iteration stops. This is useful for turning polling-style APIs (like reading fixed-size chunks or reading until a marker) into a clean iterator.
with open('data.bin', 'rb') as f:
    for chunk in iter(lambda: f.read(1024), b''):
        process(chunk)
Real-world example A file-reading loop uses iter(f.read_chunk, '') to process a stream in fixed-size blocks until an empty read signals end-of-file.

Common follow-ups: What other APIs commonly use the sentinel pattern?;How does this compare to a while-loop with a manual read?

File I/O & Context Managers;Iterators & the Iterator Protocol
All 15 Iterators & the Iterator Protocol questions