Session 11. State and memory — Mon 28 Sep

Quick quiz (ungraded)

Because dicts are not thread-safe, and the tests are single-threaded
Nothing here is a race. The same sequence on one machine gives the same wrong answer every time.
Because every test uses one user, and one user cannot read anybody else
Correct. The bug needs a second user to exist, so a suite written for one person can never fail on it. The second user is the test.
Because the values happen to be strings
The type of the value changes nothing. Two users, one key, one bucket.

None
Correct. A miss is the normal case on every user's first day, and it is an answer, not an exception.
A KeyError, so the caller knows it was never stored
The caller already knows: it got nothing. A raise on the read path takes down an answer over a case you expected.
The last value anybody stored, as a sensible default
That default belongs to another user. It answers every miss, it never raises, and it is the leak wearing a helpful face.

Because empty strings cannot be dict keys
They can. Nothing in Python stops this, which is exactly why the store has to.
Because a missing id becomes one bucket that every caller with a missing id reads and writes
Correct. It does not crash — it silently shares. Refuse it in both functions, before touching the store.
Because the value should be stored under a generated id instead
Inventing an owner for data whose owner is unknown is the same bucket with a longer name.

Nothing; the caller edited its own copy
Only if the store made a copy. Returning the stored object hands out the object itself.
What the next reader sees, without any call to remember
Correct. A returned reference is write access that never appears in the API. Copy on the way out, and on the way in.
The store, but only until the next write refreshes it
No write is coming. The edited list is the stored list, and it stays edited.

The expired one, and only it
Correct. The one with no expiry might be true or might be a year out of date, and nothing in your code can tell you which. Unaudited is not the same as fresh.
Both, since neither was written today
Age is not staleness. A locale set in 2019 may still be right; the expiry is what makes the question answerable.
Neither, because staleness is a judgement a program cannot make
It is a judgement you make once, when you store the item, and write down as a date.