Multisort Python

Question

I'm trying to write a custom exporter for Blender in Python. However, I'm having some trouble figuring out how I could use Python's native sort implementation to sort my vertices in the way I need.

Basically, I need to export the vertices in such a way where they can form a grid like this, assuming that V is a double.

V V V V V
V V V V V
V V V V V
V V V V V

The "engine" that I'm writing automatically retrieves the X and Z value of the coordinate based on where it is in the vertex map shown above. The V represents the Y coordinate. So this 5 by 5 map of vertices would create a 4 by 4 faced mesh.

However, in Blender, vertices appear to be ordered individually for each face of a mesh, rather in a format like mine. So, in order to export it, I need to order the coordinates first by depth (Y coordinate in Blender) and then by width (X coordinate in Blender). It would have to behave like a SQL query, where the first paramter given takes precedence in order compared to the second.

Here is what my code currently looks like

#!BPY

"""
Name: 'VaV Export'
Blender: 269
Group: 'Export'
Tooltip: 'Export to VaV file format'
"""
import Blender
import bpy

class Coordinate:
    def __init__(self, posX, posY):
        self.posX = posX
        self.posY = posY

    def sortX(self, other):
        if posX < other.posX:
            return 1

    def sortY(self, other):
        if posY < other.posY:
            return 1



def write(filename):
    out = open(filename, "w")
    terraindata = bpy.data.meshes["Terrain"]
    vertices = terraindata.vertices
    vertices = sorted(vertices, key = lambda x: x.co.y, reverse = True)
    vertices = sorted(vertices, key = lambda x: x.co.x, reverse = False)
    print(vertices)

Blender.Window.FileSelector(write, "Export")

The first call to sorted() successfully orders it according to depth (Y). However, The 2nd call to sorted messes up the order (as expected). How could I amend this to allow both sorts to occur without the 2nd messing up the first?

Thanks for your time in advance!

Answer 1

Change

vertices = sorted(vertices, key = lambda x: x.co.y, reverse = True)

to

vertices = sorted(vertices, key = lambda x: (x.co.y, x.co.x), reverse = True)

...and that should do it.

Answer 2

If I understand correctly, you want to order your vertices by decreasing y coordinate, with coordinates with equal y values ordered by increasing x coordinate. If that's right, you can return a tuple (-x.co.y, x.co.x) from your key function to sorted and Python will do both the whole sorting at once.

An alternative would be to take a page from radix sort and sort by least-significant coordinate (e.g. x) first, then by the more significant coordinate (y). This will work since Python's sort is stable.