MediumStateful DesignPython 3

TTL Key/Value Store

Build a deterministic in-memory store where every read path treats expired keys as missing.

35m3 sample tests5 hidden tests

Implement an in-memory Store with set, get, delete, and scan.

Requirements

Store(now) receives a zero-argument clock function.
set(key, value, ttl=None) stores a value. ttl is measured in seconds.
get(key) returns the value or None when missing or expired.
delete(key) removes a key and returns True if it existed and was not expired.
scan(prefix) returns sorted (key, value) pairs for non-expired keys whose key starts with prefix.
Expiry is inclusive: a key is expired when now() >= expires_at.

Example

python

clock = {"t": 0}
store = Store(lambda: clock["t"])
store.set("user:1", "Ada", ttl=10)
assert store.get("user:1") == "Ada"
clock["t"] = 10
assert store.get("user:1") is None

Constraints

Don't use sleep in tests or implementation.
Use standard-library Python only.

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