You are shown a Celery-based ingestion pipeline that processes user-uploaded CSVs. Under concurrent load it silently drops roughly 2% of rows with no errors logged. Explain your first three diagnostic steps, then propose a fix — the interviewer adapts its follow-ups to whatever you say.

A function appends to a list passed in as a default argument, and is called three times with no arguments. What prints on the third call, and why?

A function appends to a list passed in as a default argument, and is called three times with no arguments. What prints on the third call, and why?

def apply_proration(customer, old_plan, new_plan):
    days_left = billing_cycle_days_remaining(customer)
    credit = old_plan.daily_rate * days_left
    charge = new_plan.daily_rate * days_left
    # BUG: this runs again if the webhook retries
    invoice.add_line(charge - credit)

InvoiceFlow: live Flask + Postgres billing service

InvoiceFlow is a running Flask service, seeded with a real Postgres database, that generates monthly invoices. Its nightly proration job double-charges customers who upgrade mid-cycle. You're dropped into a live IDE with the running app and asked to find and fix it.