Level 7 · A weekly summary with built-ins

The built-ins that do the counting

Python ships with a handful of functions you will use in almost every script, no import needed:

units = [row[1] for row in week]
sum(units)      # 271
len(week)       # 6
max(units)      # 102
min(units)      # 9
sorted(units)   # [9, 15, 41, 46, 58, 102]  a new list, smallest first

sum also takes a generator directly, sum(row[1] for row in week), so you don’t need the extra list. sorted returns a new list and leaves the original alone, unlike .sort(), which reorders the list in place.

lambda: a tiny function for a key

max(units) tells you the biggest number, but not which product sold it. To get the product, give max the whole rows and a key: a function that says what to compare. A lambda writes that function inline, in one expression:

max(week, key=lambda row: row[1])   # ('Hazel Linen Throw Pillow', 102, 20.99)
min(week, key=lambda row: row[1])   # ('Tidewater Concrete String Lights', 9, 379.0)

lambda row: row[1] means “take a row, return its units”. It is the same as a def with one return line, without a name. The key can compute too, which is how you rank by revenue:

sorted(week, key=lambda row: row[1] * row[2], reverse=True)

The desk lamp sells only 15 units but at $379 it brings in $5,685, the most this week. The pillow, the best seller by units, is fifth by revenue ($2,140.98). Which “best” you report depends on the question, so name the measure.

range for numbering

range(start, stop) produces whole numbers from start up to, but not including, stop. range(1, 4) is 1, 2, 3, which is exactly the ranks of a top three:

[f"{i}. {by_revenue[i - 1][0]}" for i in range(1, 4)]

Ranks start at 1 but list positions start at 0, hence i - 1. range also takes a step: range(0, 14, 7) gives 0 and 7, the first day of each week in a two-week window. And range(len(week)) walks the positions 0 to 5, which you’ll see in older code that loops by index. Looping over the rows directly, as above, is usually clearer.

Common mistakes

• Calling max(week) with no key. Tuples compare by their first item, the name, so you get “Tidewater Concrete String Lights” because T comes last in the alphabet.
• Forgetting reverse=True, which puts the smallest revenue first.
• range(1, 3) for a top three. The stop value is excluded.