What does sorting a list of lists in python mean for the following example:

Question

>> L = [[9, 1], [3, 4]]

>> sorted(L, key=lambda x:x[0])

[[3,4], [9,1]]

The above will sort the list according to the first element, meaning that the list with the smallest first element will appear first.

However, what does the following mean?

>> L = [[9, 1], [3, 4]]

>> sorted(L, key=lambda x:(x[0], x[1]))

[[3,4], [9,1]]

Or even

>> L = [[9, 1], [3, 4]]

>> sorted(L, key=lambda x:(x[0], x[1], x[0]))

[[3,4], [9,1]]

I'm not sure what keys mean when they have more than one item.

Answer 1

Hope this makes it clear:

>>> L
[[9, 1], [3, 4], [3, 2], [9, 0]]
>>> sorted(L, key=lambda x:(x[0], x[1]))
[[3, 2], [3, 4], [9, 0], [9, 1]]

Sort by first and then second element of the sublist.