Sort List By Second (or Nth) Element In Python: 1 Line Of Code

How do you sort a list of lists by the second element using Python?

To sort a list of lists by the second element in each list use the ``` key ``` parameter to either the ``` .sort() ``` list function (if modifying the original list) or the ``` sorted() ``` function (if returning a new list).

Here’s an example demonstrating how you can do this with code:

``````>>> list_2d = [['2nd', 2], ['3rd', 3], ['1st', 1]]
>>> list_2d.sort(key=lambda x: x[1])
>>> print(list_2d)
[['1st', 1], ['2nd', 2], ['3rd', 3]]``````

As you can see from the above code the ``` key ``` parameter in the ``` .sort() ``` list function uses a ``` lambda ``` expression which takes each list and extracts the second element. This is used as the key to determine how each list will be sorted.

This approach mutates the original list.

If you need to return a new list then use the ``` sorted() ``` built-in function, like so:

``````>>> list_2d = [['6th', 6], ['4th', 4], ['5th', 5]]
>>> sorted(list_2d, key=lambda x: x[1])
[['4th', 4], ['5th', 5], ['6th', 6]]
>>> print(list_2d)
[['6th', 6], ['4th', 4], ['5th', 5]]``````

The result from the ``` sorted ``` function produces a new 2d list, as can be seen when printing the original ``` list_2d ``` – this has not changed.

Sort List By Value

With this same technique, you should be able to sort a list if lists by any index within the lists. Whether it be the second or third or… nth index within the list, the same process occurs: use the ``` key ``` parameter referencing a ``` lambda ``` function that extracts the element you want to compare across all lists .

Summary

To sort a list of lists by the second element use the ``` key ``` property in either the ``` .sort() ``` list method or the ``` sorted() ``` function which is set to ``` lambda x: x[1] ``` where ``` 1 ``` in the ``` lambda ``` function represents the nth index you would want to sort the lists by.

Ryan Sheehy
Ryan has been dabbling in code since the late '90s when he cut his teeth exploring VBA in Excel. Having his eyes opened with the potential of automating repetitive tasks, he expanded to Python and then moved over to scripting languages such as HTML, CSS, Javascript and PHP.