Ledger

A ledger is a record of deposits and withdrawals, typically used to determine the total amount of money at some point.

In this exercise, we're going to pair on implementing an in-memory ledger. We've provided an interface that we'd like you to satisfy. Your goal is to fill in the implementation!

Guidelines

For the purpose of this exercise, you will retain the ledger data in-memory, so you will not need to interact with any external databases/data stores.

You are free to consult language resources and documentation on the internet. Be sure to talk through your assumptions and don't be afraid to ask questions.

Top Level Operations

Your ledger will implement some top level operations / functions we've defined on a python class.

• balance() - Returns the current balance
• deposit(<amount>) - Adds amount to the current total balance, returns an unique entry ID referencing the deposit
• withdraw(<amount>) - Subtracts amount from the current balance, returns an unique entry ID referencing the withdrawal

We've provided a generate_entry_id function to generate an unique random ID.

There's also one additional function to implement

• balance_at(<entry-id>) - Returns the current balance up to and including the entry ID. Raises an EntryNotFoundError if the entry ID does not exist in the ledger. We've already defined this error for you.

Transactions

The ledger allows actions to be performed as part of a transaction, similar to how a database would conduct transactions.

• begin() - Start a new transaction. Transactions can be nested
• commit() - Finish all open transactions and write their actions to the ledger. Raises a TransactionError if commit() is called but there are no open transactions. We've already defined this error for you.
• rollback() - Cancel only the current transaction without writing any actions captured within that particular transaction to the ledger. Raises a TransactionError if rollback() is called but there are no open transactions. We've already defined this error for you.

The balance and balance_at functions should include any entries from deposits and withdrawals within any open transaction.