Python sum(), max() and min(): Built-in Accumulators

Python's sum(), len(), max() and min() are built-in accumulators: each runs a common loop for you and returns the result.

Read The Accumulator Pattern first. You should know how a loop builds a total or a count.

This lesson shows when one call can replace a loop, and when you still need the loop. Every block runs on its own with fixed data, and the text under it says what it prints.

Add up a list with sum()

sum() adds every number in a list. It does the same work as a total that starts at 0.

rolls = [3, 5, 2]
print(sum(rolls))
print(len(rolls))
print(sum(rolls) / len(rolls))

This prints 10, 3 and 3.3333333333333335. len() counts every item, so sum() divided by len() gives the average.

Find the biggest and smallest

max() returns the largest item and min() the smallest.

rolls = [3, 5, 2, 6]
print(max(rolls))
print(min(rolls))

This prints 6 and 2. Both also work on strings: max(["ant", "bee"]) is "bee", the last in alphabetical order.

Know the edge cases

sum([]) is 0, but max() and min() have no answer for an empty list. They raise ValueError.

print(sum([]))
try:
    print(max([]))
except ValueError:
    print("max() needs at least one item")
print(max([], default=0))

This prints 0, the error message, then 0. The default= value is what max() returns when the list is empty. Pass it whenever an empty list is possible.

Choose a loop or a built-in

Use a built-in when it is the whole task. Use a loop when each item needs a decision first, such as adding only the big rolls.

rolls = [3, 5, 2, 6]
big_total = 0
for roll in rolls:
    if roll >= 4:
        big_total = big_total + roll
print(big_total)
print(sum(roll for roll in rolls if roll >= 4))

Both lines print 11. The second passes sum() a generator expression: it reads as "each roll in rolls, kept only if roll >= 4". Use it once you can write the loop version.

Note: This lesson leaves out max() with key=, which picks the best item by a rule you choose. See Tracking the Best Candidate for the loop version.

Your turn

Print the total, the average, the highest and the lowest of scores, one per line. Use built-ins, not a loop.

scores = [8, 4, 9, 7]
# add four prints: sum, average, max, min

Expected output

Want to keep your progress? A free account saves every exercise you pass and gives AI hints when you're stuck.