Drag and drop two chromosomes with pygame

Starting to understand how to move sprites with pygame, a small result:

The following code is still not a karyotyper (no classifier). It is a model to understand how to move two sprites.

# -*- coding: utf-8 -*-
"""
@author: jean-pat
"""
#!/usr/bin/env python
#
import os, pygame
from pygame.locals import*

def load_image(name, colorkey=None):
    fullname=os.path.join("data", name)
    try:
        image=pygame.image.load(fullname)
    except pygame.error, message:
        print "Impossible de charger l'image:",name
        raise SystemExit, message
    image = image.convert()
    if colorkey is not None:
        if colorkey is -1:
            colorkey = image.get_at((0, 0))
        image.set_colorkey(colorkey, RLEACCEL)
    return image, image.get_rect()

class Ichrom(pygame.sprite.Sprite):
    def __init__(self,image,initpos):
        pygame.sprite.Sprite.__init__(self)
        self.pos = initpos
        self.image, self.rect=image
        self.button=(0,0,0)#mouse buttons not pressed
        self.selected = 0
        self.mouseover=False
        self.focused=False
        print "init chrom at ",self.pos

    def rollover(self):
        """Test if the mouse fly over the chromosome 
        self.mouseover==True if mouse flying over the chrom,
        False if not"""
        mpos=pygame.mouse.get_pos()#mouseposition
        #test if mouse roll over the sprite
        if self.rect.collidepoint(mpos):
            self.mouseover=True
        else:
            self.mouseover=False
    def update(self):
        self.button=pygame.mouse.get_pressed()
        mpos = pygame.mouse.get_pos()
        self.selected=self.rect.collidepoint(mpos)
        #the mouse flies over a chromosome
        if (self.mouseover):
            #print "mouse pos:",mpos
            if self.button==(1,0,0):     # )  
                pos = pygame.mouse.get_pos()
                self.rect.midtop = pos

def main():
    pygame.init()
    screen = pygame.display.set_mode((400,300))
    pygame.display.set_caption("Karyotyper")
    pygame.mouse.set_visible(True)
    
    background = pygame.Surface(screen.get_size())
    background = background.convert()
    background.fill((0, 0, 0))

    screen.blit(background,(0, 0))
    pygame.display.flip()
    
    i1=load_image("/home/claire/Applications/ImagesTest/jp/Jpp48/13/DAPI/particules/part15.png", -1)
    i2=load_image("/home/claire/Applications/ImagesTest/jp/Jpp48/13/DAPI/particules/part9.png", -1)
    chr1 = Ichrom(i1,(0,0))
    chr2=Ichrom(i2,(30,30))
    allsprites = pygame.sprite.RenderPlain((chr1,chr2))
    clock = pygame.time.Clock()

    while 1:
        clock.tick(60)
        for event in pygame.event.get():
            if event.type == QUIT:
                return
            elif event.type == KEYDOWN and event.key == K_ESCAPE:
                return
            if event.type ==pygame.MOUSEBUTTONDOWN:
                #need to be modified to handle a list of chromosomes
                chr1.rollover()
                chr2.rollover()
        allsprites.update()
        screen.blit(background,(0,0))
        allsprites.draw(screen)
        pygame.display.flip()
    
if __name__ == '__main__': main()