Lesson 12 — Aggregations per group

Totals per group

Sometimes you need a total for each user, not one grand total. Django does this with .values("user").annotate(total=Sum("amt")). In Python, a defaultdict(int) is handy: it is a dict where a missing key starts at 0, so you can add to it straight away.

totals = defaultdict(int)
totals["Sam"] += 50   # no KeyError

Note: the exercise runs in your browser without Django installed, so it is a small plain-Python simulation of what Django does for you.

Why it matters: "sales per product" or "posts per author" are grouped totals — reports are full of them.