What are generators and why use them?

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A **generator** produces values **lazily**, one at a time, instead of building a whole collection up front. You create one with a function that uses `yield`, or with a generator expression `(x for x in ...)`. Calling the function doesn't run its body — it returns a generator object; each `next()` runs to the next `yield`, suspends, and **remembers all local state** for the following call.

Why use them:

- **Memory** — a generator holds one item at a time, so you can process huge or even **infinite** streams (log lines, a number sequence) that would never fit in a list.
- **Composability** — generators chain into lazy pipelines (`(parse(l) for l in lines)`), doing work only as items are pulled.
- **Time-to-first-result** — you get item #1 without computing item #1,000,000.

The trade-offs: single-pass (you can't re-iterate or index it), and no `len()`. `yield from` delegates to a sub-iterator.

**Interview tip:** the killer line is 'constant memory regardless of input size.' Show a gen

What are generators and why use them?

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