[Python] Pygame Isometric Grid?

Is there an algorithm for snapping to an isometric grid?

This is the one I came up with:

def Iso(argument0,argument1):
a = round(pygame.mouse.get_pos()[1]/argument1 - pygame.mouse.get_pos()[0]/argument0);
b = round(pygame.mouse.get_pos()[1]/argument1 + pygame.mouse.get_pos()[0]/argument0);
x = (b - a)/2*argument0;
y = (b + a)/2*argument1;
return (x,y)


and it looks like this:

Anyone got any ideas??

Here is my code:

import pygame
from pygame.locals import *

pygame.init()
screen=pygame.display.set_mode((640,480))
curs=pygame.image.load('white-0.gif').convert()
curs.set_alpha(100)
g1=pygame.image.load('green-0.gif').convert()
tiles=[]

def Iso(argument0,argument1):
a = round(pygame.mouse.get_pos()[1]/argument1 - pygame.mouse.get_pos()[0]/argument0);
b = round(pygame.mouse.get_pos()[1]/argument1 + pygame.mouse.get_pos()[0]/argument0);
x = (b - a)/2*argument0;
y = (b + a)/2*argument1;
return (x,y)

class Tile(object):
def __init__(self,spr,pos1,pos2):
self.pos=(pos1,pos2)
self.spr=spr
while True:
screen.fill((90,90,0))
mse=pygame.mouse.get_pos()
for e in pygame.event.get():
if e.type==QUIT:
exit()
if e.type==MOUSEBUTTONUP:
if e.button==1:
pos=Iso(16,16)
tiles.append(Tile(g1,pos[0],pos[1]))
pos=Iso(16,16)
screen.blit(curs, (pos[0],pos[1]))
for t in tiles:
screen.blit(t.spr,t.pos)
pygame.display.update()


UPDATE: Managed to get it to work like this: Just having a few depth issues..

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