sort lists of lists with multiple criteria: similar values need to be treated equal

[stillsen]

Hey ichabod!
Thank you very much for your input, it helped a lot!

First, i defined similarity as deviation of y towards the group mean within a certain tolerance. As i do not need so precise, I defined as deviation towards the first element of the group within a certain tolerance.
I implemented it as you suggested - I first sort it according to y, then augment the list with a 4th, group common similarity value and use this as first search criteria. This group common similarity value, I defined to be the first value of such a group and to be updated if the difference exceeds a limit.

def sort_circles(circles, limit):
    circles = sorted(circles, key=lambda x: x[1])
    old_y = circles[0][1]
    for (i,(x,y,r)) in enumerate(circles):
        if abs(old_y - y) > limit:
            old_y = circles[i][1]
        circles[i] = np.append(circles[i], old_y)

    circles = sorted(circles, key=lambda x: (x[3], x[0]))
    circles = [(x,y,r) for (x,y,r,z) in circles]
    return circles