Sorting Lists

Why this matters

Leaderboards, alphabetical contact lists, cheapest-first prices: sorting is everywhere. Python can sort almost anything with one call, and you control how with a key.

The idea

nums = [3, 1, 2]
print(sorted(nums))                # [1, 2, 3]   (a new list)
print(sorted(nums, reverse=True))  # [3, 2, 1]
nums.sort()                        # sorts nums itself and returns None

• sorted(x) returns a new sorted list and leaves x alone.
• x.sort() sorts x in place and returns nothing.

The key says what to sort by. It is a function that is applied to every item:

words = ["pear", "Fig", "apple"]
print(sorted(words))                 # ['Fig', 'apple', 'pear']   capitals sort first!
print(sorted(words, key=str.lower))  # ['apple', 'Fig', 'pear']
print(sorted(words, key=len))        # ['Fig', 'pear', 'apple']

Python’s sort is stable: items that tie keep their original order.

To print a list as text, join the items with a separator: ", ".join(["a", "b"]) is "a, b" (every item must be text).

Try it

Sort [10, 9, 100, 1] as numbers, then sort ["10", "9", "100", "1"] as text. Why are the answers different?

Your turn

The starter reads names separated by spaces into a list called names. Print three lines, each with the names separated by a comma and a space (the example is for bob alice Carol dave):

Sorted: alice, bob, Carol, dave
By length: bob, dave, alice, Carol
Reverse: dave, Carol, bob, alice

The first and last ignore capital letters. “By length” is shortest first.

Recap

• sorted() returns a new list; .sort() changes the list and returns None.
• key= chooses what to sort by; reverse=True flips the order.
• ", ".join(items) turns a list of strings into one string.

Common mistakes

• names = names.sort() sets names to None. Use sorted(...) or call .sort() on its own line.
• Forgetting capitals sort before lowercase with the default sort.
• Joining numbers. ", ".join([1, 2]) is a TypeError: convert with str() first.