Sorting: sorted(), Keys & Custom Order

14 min 35 XP

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Step 1

Learn the idea

You'll rarely write a sorting algorithm: Python's sorted() (Timsort, invented for Python, adopted by Java and Android) is world-class. The real skill is telling it what order means:

sorted(words): alphabetical
sorted(words, key=len): by length; the key function is called on each item and the results are compared instead
sorted(words, key=lambda w: (len(w), w)): by length, ties broken alphabetically, tuples compare position by position
reverse=True: flip any of the above

The tuple-key trick is the professional move: 'sort by department, then salary descending, then name' is one line in Python.

Where you'll use this

Every table header you've ever clicked runs a key-based sort. Timsort, invented for Python, now also sorts Java and Android. The tuple-key trick handles real reporting: department, then salary, then name.

Common mistakes

Confusing sorted(items) (returns a new list) with items.sort() (in place, returns None): printing the result of .sort() gives None.
Sorting strings expecting numeric order: "10" < "9" alphabetically. Convert first or use key=int.
Reaching for cmp-style comparison logic: Python only does key functions, which are simpler anyway.

Pro tip

Sort descending on one field and ascending on another by negating numbers: key=lambda r: (-r["score"], r["name"]).

Step 2

Try it yourself

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The lesson example is loaded and ready: press Run, then change something and run it again. Breaking it is part of learning. Want a clean slate? Tap “New blank”.

PYexample.py

⌘ + Enter to run

Output appears here…

Step 3

Pass the challenge +35 XP

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For words = ["banana", "fig", "cherry", "kiwi", "apple"]: first print them sorted alphabetically, joined by ", ". Then print them sorted by length with alphabetical tie-breaks, joined the same way.

Target output

apple, banana, cherry, fig, kiwi
fig, kiwi, apple, banana, cherry

PYchallenge.py

words = ["banana", "fig", "cherry", "kiwi", "apple"]

print(", ".join(sorted(words)))
print(", ".join(sorted(words, key=lambda w: (len(w), w))))

Run your code to check it…

Step 4

Check your understanding

1. What does the key argument of sorted() do? Encrypts the list Transforms each item into the value actually compared Removes duplicates Speeds up sorting

2. How do tuples like (len(w), w) compare? By total size Position by position: later positions break earlier ties Randomly They can't be compared

Last step

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