lesson 70 Built-ins & Sorting · intermediate

Totals and extremes with sum, min and max

`sum` adds a collection up; `min` and `max` find its extremes. With `key` they return the whole winning record, and with `default` they survive an empty input.

You'll be able to

Total a field with sum(x["field"] for x in items)
Find the record with the smallest or largest field using min(items, key=…) and max(items, key=…)
Handle an empty input with default= instead of letting ValueError stop the program

The idea

Totals and extremes are the most common questions in reporting: the invoice subtotal, the cheapest product, the fastest lap. sum, min and max each take a collection, often a generator expression, and return one value.

Run each example to see what it prints. Running examples doesn't count toward progress.

example-1.py · An invoice summary

lines = [
    {"sku": "SPO-11681", "qty": 2, "price": 7.5},
    {"sku": "HOM-22847", "qty": 1, "price": 31.0},
    {"sku": "ELE-24781", "qty": 1, "price": 171.25},
]
units = sum(line["qty"] for line in lines)
subtotal = sum(line["qty"] * line["price"] for line in lines)
print(units, subtotal)
print(min(line["price"] for line in lines), max(line["price"] for line in lines))

min and max of nothing has no answer, so Python stops with an error rather than inventing one.

example-2.py · The largest of nothing

delays = []
print(max(delays))

To get the record rather than the number, pass the records and a key, exactly as with sorted. On a tie, the first record wins. default= is returned when the input is empty, which turns *no data* into a value you can test, such as None.

example-3.py · Fastest lap, slowest driver, no laps

def lap_seconds(lap):
    return lap["seconds"]

laps = [
    {"driver": "Ana", "seconds": 92.4},
    {"driver": "Ben", "seconds": 91.8},
    {"driver": "Chen", "seconds": 91.8},
]
best = min(laps, key=lap_seconds)
print(best["driver"], best["seconds"])
print(max(laps, key=lap_seconds)["driver"])
print(min([], key=lap_seconds, default=None))

When to use it: any total, smallest or largest. Mistakes to avoid: expecting max(x["price"] for x in items) to give you the product (it gives the price; use key for the product); min/max on something that may be empty without default; and sum of texts, which raises TypeError. A tuple key settles ties on purpose: min(routes, key=minutes_then_price).

Review

sum(...) totals; min/max return the extreme value, or the extreme record with key=.
On a tie, min and max return the first item; a tuple key settles ties on purpose.
min/max of nothing raise ValueError unless you pass default=.
Next module: functions that take any number of arguments, keyword-only options, small inline functions, closures and recursion.

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