Sorting with a Key Function

You've used sorted() to sort numbers and strings. The key= parameter lets you control what each item is sorted by.

Sort a list of words by their length instead of alphabetically:

words = ['fig', 'kiwi', 'apple', 'cherry']
result = sorted(words, key=len)
print(result)

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Output

The key= parameter accepts any callable. Python applies it to each item before comparing:

sorted(iterable, key=callable)
sorted(iterable, key=callable, reverse=True)

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What will be the output?

words = ['fig', 'kiwi', 'apple', 'cherry']
result = sorted(words, key=len)
print(result[0])

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Use key=str.lower for a case-insensitive sort. This tells Python to call str.lower() on each item before comparing — it doesn't change the items themselves:

names = ['charlie', 'Alice', 'BOB']
result = sorted(names, key=str.lower)
print(result)

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Output

When you write key=str.lower, you pass the method as a reference (no parentheses). Python calls str.lower(item) on each item internally.

What will be the output?

names = ['bob', 'Alice', 'CHARLIE']
result = sorted(names, key=str.lower)
print(result)

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Use key=abs to sort numbers by their absolute value:

nums = [-3, 1, -5, 2, 4]
result = sorted(nums, key=abs)
print(result)

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Output

What will be the output?

nums = [-3, 1, -2, 4]
result = sorted(nums, key=abs)
print(result)

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Combine key= with reverse=True to sort in descending order by your chosen key.

Sort words from longest to shortest:

w = ['fig', 'kiwi', 'apple', 'cherry']
result = sorted(w, key=len, reverse=True)
print(result)

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Output

What will be the output?

w = ['fig', 'kiwi', 'apple', 'cherry']
result = sorted(w, key=len, reverse=True)
print(result[0])

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What will be the output?

words = ['AA', 'a', 'abc']
result = sorted(words, key=len)
print(result[1])

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What will be the output?

names = ['alice', 'Bob', 'CHARLIE']
result = sorted(names, key=str.lower)
print(result[-1])

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