I recently ran into a situation where had a list of strings I wanted sorted in a particular way in Python. The sorted() method offers a quick way to sort a collection, but in my case I needed to do more than just sort alphabetically.

The following is a partial example of a list of strings that represent a table of contents that need sorting.

items = [
	"10. Chapter 10",
	"11. Chapter 11",
	"8. Chapter 8",
	"9. Chapter 9",
]

The above shows the strings out of order for demonstration purposes. In my case the list is populated from a call to os.listdir() and came in being out of order to start with.

To address this issue the Python sorted() method supports the ability to define a key to sort against. Furthermore that key can be a lambda expression. This means that I can somehow parse the incoming string to get only the numeric part at the beginning and use that as the key to sort on.

items = sorted(items, key=lambda item: int(item.partition(".")[0]))

The magic is in two parts here. The first is the lambda expression. This defines an inline function used to provide the sorted() method a key to sort on. Next I mentioned that we need to get only the numeric part of the string. The partition() method of the string object in Python will split a string up based on the first occurrence of ”.” found in the string. Then we cast the number found as an integer, and we have a key. Tada!

Hope this was helpful. Happy coding!