Lesson 23 — Transactions & rollback

Transactions: all or nothing

A transaction groups several changes so they either all happen (COMMIT) or none do (ROLLBACK). Moving money is the classic case: subtract from one account and add to another. If the second step fails, the first must be undone, or money vanishes. In Python, with db: starts a transaction that commits if the block finishes and rolls back if an error is raised.

with db:
    db.execute("UPDATE ...")
    raise ValueError("oops")   # -> everything in the block is undone

Why it matters: banks, shops and booking systems rely on transactions to keep data consistent when things go wrong.