Sorting iterables PREMIUM

How can you sort things in Python?

The list sort method

Python's lists have a sort method, which will modify the list to order all of its items from smallest to largest:

>>> numbers = [4, 2, 7, 1, 5]
>>> numbers.sort()
>>> numbers
[1, 2, 4, 5, 7]

The sort method relies on the less-than (<) operator.

>>> "apple" < "banana"
True

So any object that works with the less-than operator can be sorted:

>>> fruits = ["apple", "kumquat", "banana", "blueberry", "kiwi"]
>>> fruits.sort()
>>> fruits
['apple', 'banana', 'blueberry', 'kiwi', 'kumquat']

But the sort method modifies lists in place, instead of returning a new list. And it only works on lists, because it's a method that lives on the list class.

The built-in sorted function

Python's built-in sorted function is a more generic version of the list.sort method.

>>> numbers = [4, 2, 7, 1, 5]
>>> sorted_numbers = sorted(numbers)
>>> sorted_numbers
[1, 2, 4, 5, 7]

The sorted function works on any iterable, so we could sort a generator object:

>>> squares = (n*n for n in numbers)
>>> sorted(squares)
[1, 4, 16, 25, 49]

Or we could even sort lines in a file:

>>> with open("my_file.txt", mode="rt") as my_file:
...     sorted(my_file)
...
['And this is the last line.\n', 'I quite like this file\n', 'This is my file!\n']

And since sorted doesn't modify the passed-in iterable, if you just need to sort for the sole purpose of looping one time, you could call sorted and then immediately loop over the result:

>>> numbers = [4, 2, 7, 1, 5]
>>> for n in sorted(numbers):
...     print(n)
...
1
2
4
5
7

This use of sorted makes it feel a little bit more like a looping helper, like the built-in enumerate, reversed, and zip functions.

Sorting in reverse (descending order)

What if you want to sort from biggest to smallest?

The sorted function accepts an optional reverse argument. When it's True, it'll sort in descending order:

>>> numbers = [4, 2, 7, 1, 5]
>>> sorted(numbers, reverse=True)
[7, 5, 4, 2, 1]

In fact, the list.sort method also accepts that reverse argument. If you give it a True or truthy value, it sorts in descending order as well.

Sorting by a "key" function

What if you wanted to sort by something that wasn't quite the original item?

For example, let's say we'd like to sort a list of strings, but not by the characters in the string. Instead, we'd like to sort them by their length.

Well, the sorted function accepts an optional key argument. That key argument should be a callable (i.e. something like a function).

>>> help(sorted)
Help on built-in function sorted in module builtins:
sorted(iterable, /, *, key=None, reverse=False)
    Return a new list containing all items from the iterable in ascending order.
    A custom key function can be supplied to customize the sort order, and the
    reverse flag can be set to request the result in descending order.

Sorted will call that key function on each item in the given iterable, and it'll sort the items by the result of whatever that key function returns for each item.

So if we passed in the built-in len function as our key function, sorted will call len on each item, and then it'll use that value (the length of each string) as the key to sort by:

>>> sorted(fruits, key=len)
['kiwi', 'apple', 'banana', 'kumquat', 'blueberry']

So you can see we're sorting these strings in order of their length, in ascending order.

Use sorted to sort any iterable

You can use the built-in sorted function to sort any iterable in Python.