Resample daily orders into a monthly summary

Question

Ops wants monthly totals from a daily order log, including months where nothing happened.

Write `monthly_totals(df)` that returns a Series indexed by month-end dates with the summed `amount` per month, and `0.0` for months with no orders.

Accepted Answer

Step 1: `resample` needs a DatetimeIndex, so `set_index('order_date')` comes first and `sort_index()` guards against unordered input. Step 2: `resample('ME')` buckets by month end and, unlike `groupby(month)`, it inserts the empty February bucket for you. Step 3: `.sum()` on an empty bucket already yields `0.0`; the `fillna(0.0)` is belt-and-braces for other aggregations.

Resample daily orders into a monthly summary

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