Sorting the order of entire columns in numpy

Question

How can I sort this 3,20 ndarray by column? The np.sort() doesnt seem to do what I think it does. I want to turn this:

                                                                 a
array([[0, 0, 0, 0, 1, 1, 1, 2, 2, 3, 0, 0, 0, 1, 1, 2, 0, 0, 1, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 2, 2, 2, 3],
       [0, 1, 2, 3, 0, 1, 2, 0, 1, 0, 0, 1, 2, 0, 1, 0, 0, 1, 0, 0]])

into this:

                                   a
array([[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 2, 2, 2, 3],
        0, 0, 0, 0, 1, 1, 1, 2, 2, 3, 0, 0, 0, 1, 1, 2, 0, 0, 1, 0],
        0, 1, 2, 3, 0, 1, 2, 0, 1, 0, 0, 1, 2, 0, 1, 0, 0, 1, 0, 0]])

Note: the columns are kept intact - see column a. They are sorted first by the first element in the column, then the second, then the third.

Thanks

Answer 1

Maybe you could use lexsort?

>>> arr[:,np.lexsort(arr[::-1])]
array([[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 2, 2, 2, 3],
       [0, 0, 0, 0, 1, 1, 1, 2, 2, 3, 0, 0, 0, 1, 1, 2, 0, 0, 1, 0],
       [0, 1, 2, 3, 0, 1, 2, 0, 1, 0, 0, 1, 2, 0, 1, 0, 0, 1, 0, 0]])

Answer 2

Make each line a tuple and use it as a sort criterion:

a = np.array([[0, 0, 0, 0, 1, 1, 1, 2, 2, 3, 0, 0, 0, 1, 1, 2, 0, 0, 1, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 2, 2, 2, 3],
       [0, 1, 2, 3, 0, 1, 2, 0, 1, 0, 0, 1, 2, 0, 1, 0, 0, 1, 0, 0]])

np.array(sorted(a, key=tuple))

Out:

array([[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 2, 2, 2, 3],
       [0, 0, 0, 0, 1, 1, 1, 2, 2, 3, 0, 0, 0, 1, 1, 2, 0, 0, 1, 0],
       [0, 1, 2, 3, 0, 1, 2, 0, 1, 0, 0, 1, 2, 0, 1, 0, 0, 1, 0, 0]])