Breaking ties with stable sorting¶

What happens when two items are “tied” in the sort order? For example, suppose we sort a list of words by their lengths. Which five letter word will appear first?

The answer is that the python interpreter will sort the tied items in the same order they were in before the sorting. This is called “stable” sorting. Not all sorting algorithms have this stability property, but python’s interpreter is required to use a stable sorting algorithm.

Stable sorting can be useful. For example, suppose that we want to sort a list of tuples based on the first element of the tuple, in ascending order. For a pair of tuples where the first items are equal, we’d like to sort them based on their second items, in descending order. How can we do that? Perhaps it’s a little counter-intuitive, but we first sort based on the secondary (tie-breaking) element, and then sort based on the primary element. The last sort is what matters, but when there’s a tie during the last sort, we get the order defined previously.