30. Sorting and key functions

`sorted(iterable)` returns a new sorted list; `list.sort()` sorts in place. Both accept a `key` function and a `reverse` flag.

python
txs = [{"amount": 5}, {"amount": 1}, {"amount": 3}]
sorted(txs, key=lambda t: t["amount"])
sorted(txs, key=lambda t: t["amount"], reverse=True)

from operator import itemgetter
sorted(txs, key=itemgetter("amount"))

Python's sort is stable — elements that compare equal keep their relative order — which matters when sorting by multiple criteria in sequence (sort by the least significant key first).

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