Monthly Active Users

The first number every product team asks for, and the first place people accidentally count events instead of people.

Write monthly_active_users(df). The input has user_id and timestamp columns. Return a DataFrame with:

• month: the calendar month as a "YYYY-MM" string
• active_users: how many distinct users had at least one event that month

Sorted by month ascending, with a clean 0..n index. A user active on twenty days in January counts once for January, and counts again in February if they come back.