Bank: whole balances refused, ranking ties reversed, expired holds kept and merged accounts losing transfers

Context

Welcome to the payments team. The bank in bank/ implements all four levels described in README.md: accounts and payments, the top spenders ranking, transfers that wait to be accepted, and merging accounts. Since the last release, four reports have come in.

Fix as much as you can in 40 minutes. The checks are graded one by one, so each report you fix counts even if the rest is not done.

BANK-11: A whole balance cannot be sent

Report

acc1 holds exactly 250. pay(..., "acc1", "acc2", 250) returns None, and so does a transfer request for 250. Paying 249 works.

Acceptance criteria

• pay and request_transfer accept an amount equal to the source's balance, leaving it at 0.
• An amount above the balance is still refused.

BANK-12: Ranking ties come out backwards

Report

Three accounts a, b and c have each paid out 100. top_spenders lists them as c(100), b(100), a(100).

Acceptance criteria

• Equal totals are listed in account id order, plain string order, with higher totals still first.

BANK-13: Expired transfers keep the money

Report

A transfer of 400 that nobody accepted expired after its hour, but the 400 never came back to the sender's balance.

Acceptance criteria

• When a pending transfer expires, its held money is back in the source account for every call from the expiry time on.

BANK-14: Merged accounts lose incoming transfers

Report

acc3 sent a transfer to acc2, then support merged acc2 into acc1. acc1 cannot accept the transfer. When a new acc2 was opened later, it could.

Acceptance criteria

• After a merge, pending transfers sent to the closed account go to the surviving account, which can accept them.
• An account opened later under the old id has no claim on them.

Notes

• Run pytest tests/visible. Three tests fail on the current code; not every report has a failing test yet.
• 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.