How to sort list of lists in Python?

Question

I have a nested list

a=[[x1,y1],[x2,y2],[x1,y2],[x2,y1],[x7,y7],[x1,y3]]

I want to sort it as:

a=[[x1,y1],[x1,y2],[x1,y3],[x2,y1],[x2,y2],[x7,y7]]

I tried to use lambda function but it wouldn't compile:

a=sorted(a,lambda x,y:x==y?x[0]+x[1],x[0])

How do I use sorted to get my desired output?

Answer 1

You could use actual Python syntax, such as a conditional expression:

sorted(a, key=lambda x: x[0] + x[1] if x == y else x[0])

This is based on a interpreting your ? syntax as the Javascript conditional expression syntax.

This of course assumes there is a separate y variable defined somewhere:

>>> a = [[1,1], [2,2], [1,2], [2,1], [7,7], [1,3]]
>>> y = [1, 1]
>>> sorted(a, key=lambda x: x[0] + x[1] if x == y else x[0])
[[1, 2], [1, 3], [1, 1], [2, 2], [2, 1], [7, 7]]

Answer 2

I believe sorted(a) will give what you want, right out-of-the box:

>>> a = [[1,1], [2,2], [1,2], [2,1], [7,7], [1,3]]
>>> sorted(a)
[[1, 1], [1, 2], [1, 3], [2, 1], [2, 2], [7, 7]]