Add Nested Begin, Commit, and Abort Semantics to a Shopping Cart

Problem Statement

Design an in-memory transactional shopping cart class, TransactionalCart, that supports nested transactions. Nested transactions allow operations to be staged, committed to parent scopes, or rolled back cleanly without affecting outer scopes or the base cart state.

Requirements

Implement the following methods in TransactionalCart:

• add_item(item_id: str, quantity: int) -> None: Adds quantity units of item_id. quantity will always be a positive integer.
• remove_item(item_id: str, quantity: int) -> None: Removes quantity units of item_id. If quantity is greater than or equal to the current quantity of item_id, the item is removed entirely. If item_id does not exist in the cart, raise a KeyError.
• get_quantity(item_id: str) -> int: Returns the current quantity of item_id visible in the current transaction scope. Returns 0 if the item is not present.
• begin() -> None: Starts a new nested transaction scope.
• commit() -> None: Merges the changes from the current nested transaction into the parent transaction scope (or base state if it was the outermost transaction). Raises an InvalidTransactionError if no transaction is active.
• abort() -> None: Discards all modifications made in the current transaction scope, restoring state to the moment the corresponding begin() was called. Raises an InvalidTransactionError if no transaction is active.

All cart modifications must immediately be visible to subsequent queries in the current scope and any child scopes opened afterwards.

Example

cart = TransactionalCart()
cart.add_item("apple", 3)
assert cart.get_quantity("apple") == 3

cart.begin() # Depth 1
cart.add_item("apple", 2)
cart.add_item("banana", 5)
assert cart.get_quantity("apple") == 5

cart.begin() # Depth 2
cart.remove_item("apple", 4)
assert cart.get_quantity("apple") == 1
cart.abort() # Discards Depth 2 changes

assert cart.get_quantity("apple") == 5
assert cart.get_quantity("banana") == 5

cart.commit() # Commits Depth 1 changes to base state
assert cart.get_quantity("apple") == 5
assert cart.get_quantity("banana") == 5

Constraints

• Nested transaction depth can reach up to 1,000 levels.
• Total operations (reads, writes, transaction controls) will be up to 100,000.
• item_id is an alphanumeric string of length 1 to 32 characters.
• quantity is an integer where 1 <= quantity <= 10^6.