How do you sort with a custom key, and how does sorted differ from sort?

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Both sort with the optional **`key=`** function — it's called once per element to derive the value to compare (a *decorate-sort-undecorate*, so it's efficient). `reverse=True` flips the order.

- **`list.sort()`** sorts the list **in place** and returns **`None`** (a common bug: `x = mylist.sort()` gives `None`). It only exists on lists.
- **`sorted(iterable)`** returns a **new sorted list** and accepts **any iterable** (a generator, dict keys, a set, a string).

Common keys: `key=len`, `key=str.lower` (case-insensitive), `key=lambda p: p['age']`, and **tuple keys for multi-level sorts** — `key=lambda p: (p.dept, -p.salary)` sorts by department ascending then salary descending. Both sorts are **stable** (equal keys keep their original relative order), which is what makes layered sorting by repeated passes work.

**Interview tip:** nail the two differences — `sort()` mutates and returns `None` (list-only) vs `sorted()` returns a new list (any iterable) — and show a **tuple key** for mul

How do you sort with a custom key, and how does sorted differ from sort?

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