Banking System

Background. A bank holds accounts, each under a string id with an integer balance. Money is added to accounts and moved between them, and the bank ranks its accounts by how much money each has sent. Every call is told the time at which it is made.

Implement the class Bank in bank.py, with the methods open_account, add_funds, move_funds, balance and top_outflow.

• open_account(now, account_id) opens an account under account_id with balance 0 and returns True. If the bank already holds an account with this id, it returns False.
• add_funds(now, account_id, amount) adds amount to the account's balance and returns the new balance.
• move_funds(now, from_id, to_id, amount) moves amount from the account from_id to the account to_id and returns the sender's new balance. It is refused when from_id and to_id are the same id or when the sender's balance is below amount; sending exactly the whole balance is allowed.
• balance(now, account_id) returns the account's balance.
• top_outflow(now, n) ranks the accounts by money sent: the total of the amounts each has sent with successful move_funds calls. It returns the first n as (account_id, money_sent) tuples, the largest first, ties broken by account id in ascending Python string order ("Zed" before "amy", "a10" before "a9"). Every account the bank holds is ranked, one that has sent nothing with 0; with fewer than n accounts, all of them are returned.

add_funds, move_funds and balance return None when an id they are given names no account. A refused call, one that returns False or None, changes nothing.

now is the time of the call in milliseconds: a positive integer up to 10^15, larger on every call than on the one before. No answer here depends on it.

Example

b = Bank()
b.open_account(1, "ines")              # True
b.open_account(2, "tomas")             # True
b.open_account(3, "ines")              # False: the bank already holds "ines"
b.add_funds(4, "ines", 700)            # 700
b.move_funds(5, "ines", "tomas", 260)  # 440: what "ines" has left
b.move_funds(6, "tomas", "ines", 300)  # None: "tomas" holds only 260
b.balance(7, "tomas")                  # 260
b.top_outflow(8, 5)                    # [("ines", 260), ("tomas", 0)]

Notes

• Account ids are non-empty strings of any characters, and case-sensitive. Amounts are positive integers of any size, and n is at least 1. Inputs are always valid.
• All 14 tests in tests/test_part1.py fail on the starter.

Done when Submit goes green.