Design a Transactional Key-Value Store

Design a Transactional Key-Value Store

Design an in-memory key-value store class TransactionStore that supports standard CRUD operations alongside transaction management. The store must support nested transactions, where changes made within a sub-transaction are only visible to the parent transaction until committed.

Requirements

1. put(key, value): Associates a value with a key. If a transaction is active, the change must be scoped to the current transaction.
2. get(key): Returns the value associated with the key. It must reflect the most recent state visible to the current transaction (checking the current scope, then parent scopes, then the global store).
3. delete(key): Removes the key. If inside a transaction, the deletion must be tracked so it is effectively hidden or removed upon commit.
4. begin(): Starts a new transaction scope. Transactions can be nested.
5. commit(): Flushes changes from the current scope into the parent scope. If at the root level, changes are finalized.
6. rollback(): Discards all changes made in the current transaction scope.

Constraints

• The store should handle arbitrary nesting levels.
• A get operation must return a special indicator (e.g., None) if the key does not exist or has been deleted in the current scope.

Example

store = TransactionStore()
store.put("a", 1)
store.begin()
store.put("a", 2)
print(store.get("a")) # Expected: 2
store.rollback()
print(store.get("a")) # Expected: 1

Edge Cases

• Deleting a key that was added in a parent transaction.
• Committing an empty transaction.
• Calling rollback() at the base level (should be ignored or raise an error).