Sort a nested list in python

I am trying to sort a nested list in Python in O(n log n) time. I am not sure how to go about it, some help would be appreciated. The inner lists are being sorted by its sum.

My list:

[10, 7, [4,5], [1,2]]

My desired output:

[[1,2], 7, [4,5], 10]

input_list = [10, 7, [4,5], [1,2]]
input_list.sort(key = lambda x: sum(x) if type(x) == list else x)
[[1, 2], 7, [4, 5], 10]