Iterators

The iterator protocol is dead simple:

• __iter__() returns an iterator (often self)
• __next__() returns the next item, or raises StopIteration

Every for-loop, comprehension, and sum()/min()/max() builds on this.

itertools is your friend

• chain, islice, cycle, count, takewhile, dropwhile
• groupby, combinations, permutations, product

Generators vs hand-rolled iterators

Generators (with yield) are usually clearer for one-off iteration. Build a class only when you need state or methods beyond __next__.

Try it

1. Write a hand-rolled iterator for the Fibonacci sequence.
2. Use itertools.product to enumerate every (rank, suit) of a card deck.