Iterators and the for loop

A for loop works on anything iterable. Under the hood it calls iter() to get an iterator, then calls next() on it again and again until the iterator raises StopIteration. Build your own iterator by giving a class __iter__ and __next__. Iterators are lazy: they hand out one value at a time instead of building a whole list.

How it reads

• iter(colors) gives an iterator; each next() hands out one item
• next(it, "done") returns the default instead of raising when it runs out
• __next__ raising StopIteration is how a loop knows to stop

Cloud tip: An iterator can only be walked once. Call list(...) if you need to loop over the values twice.