Transactional Key-Value Store

Grow a plain dictionary into a store with value counts, expiring keys, nested transactions and atomic conditional batches.

Step 1 — Get, set, delete, count

Build the core store. Every method takes a now argument — an integer clock in seconds that is non-decreasing across calls. Ignore it for this step; later steps need it.

class Store:
    def set(self, key, value, now=0) -> None
    def get(self, key, now=0) -> value | None
    def delete(self, key, now=0) -> bool
    def count(self, value, now=0) -> int

Rules:

get returns None for a key that is not present.
delete removes the key and returns True, or returns False if it was not there.
count(value) returns how many keys currently hold exactly that value.
count is called far more often than set; it should not scan the whole store.

s = Store()
s.set("a", 10)
s.set("b", 10)
s.count(10)     # 2
s.set("a", 20)
s.count(10)     # 1
s.delete("b")   # True
s.delete("b")   # False
s.get("b")      # None