Practice problem · Rippling · Medium

Expense Rule Engine

A rule engine for a corporate card product. 4 stages in Python with an AI assistant. Pro.

Stage 1 of 4: Fix the amount comparison bug

Background. A rule engine for a corporate card product. Employees submit expenses, managers choose policies, and the engine flags violations.

policy_rules.py defines usd_over_limit, used by several expense policies. The starter has a bug. One test in tests/test_part1_amounts.py fails.

Find it and fix it. Don't change the public signature.

Done when Submit goes green.

Constraints

  • Python 3.12+ standard library only
  • Time limit: 10 seconds per test
  • Editable: policy_engine.py, policy_rules.py
  • Read-only: all tests/test_*.py files

What comes after stage 1

The next stages open once the tests for the one before them pass. Each raises the bar, the way an interviewer does in the room.

  1. Stage 2: Flag per-expense violations.
  2. Stage 3: Aggregate trip-level violations.
  3. Stage 4: Composite predicates.

The files you get

  • Yours to edit: policy_engine.py, policy_rules.py.
  • Read-only: tests/__init__.py.
  • Tests you can read and run: tests/test_part1_amounts.py. Hidden tests run on Submit.

The round: an agent in the editor

The assistant here works like the one in the interview IDE. It reads your files, runs code and proposes changes, and you accept or reject each one. It doesn't volunteer ideas, so what gets built is what you asked for. Along the way the interviewer asks about your decisions, and at the end the report says what you checked and what you accepted unread.

Frequently asked, briefly answered

Is Expense Rule Engine free?

It is a Pro problem. Free plan includes one complete problem, plus the first stage of several more, so you can see how a round works before you pay.

Which company asks something like this?

Rippling.

Do I bring my own AI tool?

No. The assistant is in the editor, same as in the interview IDEs at Meta, Stripe and Amazon. What you choose at Begin is the format: an agent that proposes changes you accept or reject, an answer-only assistant that can't write code, or no assistant at all.

What does the interviewer ask?

4 questions along the way about your own code, and one at the end about the whole session. A weak answer gets one follow up. The answers go into the report, which ends with a hire or no hire verdict.

Start the problem

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