Question

It is easy to implement a regular double sort:

pairs = [(1, 2), (2, 1), (1, 3), (2, 4), (3, 1)]

sorted(pairs,key=lambda x: (x[0],x[1]))
# out: [(1, 2), (1, 3), (2, 1), (2, 4), (3, 1)]

I am interested how to do it with the second elements in the reverse order. This can be easily implemented by grouping the pairs by the first item at first and then adding the the sorted second items together. I have implemented this both using itertools.groupby and defaultdict. Still, it remains far more complex, than the regular double sort, so i wonder, if there is a neat trick to do it in a more concise way.

double_sort(pairs)
# out: [(1, 3), (1, 2), (2, 4), (2, 1), (3, 1)]

PS! I know how to do it with numpy.argsort and would mostly like to see a standard lib approach.

Answer 1

This will work for numbers and similar data types

sorted(pairs, key=lambda x: (x[0], -x[1]))

This will work for all comparable types only in Python 2 🙁

sorted(pairs, lambda x, y: cmp(x[0], y[0]) or cmp(y[1], x[1]))