Bank: open accounts, move money and rank the top spenders

Context

Welcome to the payments team. We are building the money-movement core of a small payments product. Its public API in bank/bank_system.py is agreed and stubbed out, and the rules are in README.md.

Build levels 1 and 2 below in 30 minutes. The checks are graded one by one, so every method you get working counts even if the rest is not done.

BANK-1: Accounts and payments (level 1)

Acceptance criteria

• create_account opens an account with a balance of 0 and refuses an id that is already in use.
• deposit returns the new balance, or None for an unknown account.
• pay moves money between two different accounts at once and returns the source's new balance. It returns None and moves nothing when an account is unknown, both ids are the same, or the source holds less than the amount.
• get_balance returns the balance, or None for an unknown account.

BANK-2: Top spenders (level 2)

Acceptance criteria

• top_spenders(timestamp, n) lists up to n accounts as "account_id(total)", by money paid out, highest first.
• Equal totals are listed in account id order, plain string order. Accounts that have paid nothing count, with a total of 0.
• Deposits and money received do not count.

Notes

• Timestamps do not matter yet; later levels use them. Keep them as the first argument.
• Run pytest tests/visible. The hidden checks call the same methods with longer sequences of calls.
• Practice task modelled on candidate reports of Meta's four-level bank system online assessment. Original StackYard code, not the actual question; not affiliated with Meta.