How to order a list of vertices based upon position?

Question

I have a list of vertices:

But the list looks like this: [0, 1, 2, 3, 4, 5, 6, 7].

I need a script to sort the list to be like the order in the loop itself: [3, 2, 1, 0, 4, 5, 7, 6].

Answer 1

What list are you referring to?

the numbers shown in the image are indexes of each vertex in the obj.data.vertices list; a list of these indexes: [0,1,2,3,4,5,6,7] (8 vertices).

In the edges in obj.data.edges list you'll find the indexes of vertices that define the edge; sample: [[0,1],[1,2],[2,3],[0,4],[4,5],[5,7],[6,7]] (8 edges).

It feels like you want to reorder edges so you can loop them based on their index in obj.data.edges; sample: [[3,2],[2,1],[1,0],[0,4],[4,5],[5,7],[7,6]].

In that case you'll need to search for a start vertex the loop. A vertex which belongs to only one edge is a fitting approach. There are 2 of those in your sample, in this code the first appearing in obj.data.vertices list is selected:

vecount = []
for e in obj.data.edges:
    while(len(vecount)-1 < max(e.vertices[0], e.vertices[1])):
        vecount.append(0)
    vecount[e.vertices[0]] += 1
    vecount[e.vertices[1]] += 1    
startidx = vecount.index(1)

Now obj.data.vertices[startidx] is the starting point of the loop Now you can create an array of edges, sorted from starting point towards end point:

edges = []
doneidxs = [startidx]
curridx = startidx
while (len(doneidxs ) != len(obj.data.vertices)):  
    for e in obj.data.edges:
        if e.vertices[0]==curridx or e.vertices[1]==curridx:
            if not e.vertices[0] in doneidxs:
                toidx= e.vertices[0]
                break;
            else:
                if not e.vertices[1] in doneidxs:
                    toidx= e.vertices[1]    
                    break;       
    edges.append([curridx, toidx])
    doneidxs.append(toidx)
    curridx = toidx

Now you can apply the new order of edges by updating vertex indexes in the edge vertices arrays:

for e in obj.data.edges:
    e.vertices[0] = edges[e.index][0]
    e.vertices[1] = edges[e.index][1]
obj.data.update()

Run this code having the mesh selected (3D View, OBJECT mode), starting with pointing out that's the object you want the code to work on:

obj = context.active_object
...(above code)...