ejemplo de arrastre con pygame

ejemplo.py

import pygame
import sys

OLPC_SCREEN_SIZE = (640, 480)

BLANCO = (255, 255, 255)
AZUL = (0, 0, 255)
AMARILLO = (255, 255, 0)

class Main(object):
    screen = None

    def run(self):
        pygame.init()
        self.screen = pygame.display.set_mode(OLPC_SCREEN_SIZE) 
        self.fondo = pygame.Surface(OLPC_SCREEN_SIZE)
        self.fondo.fill(BLANCO)
        self.screen.blit(self.fondo, (0,0))

        cuadro_azul = pygame.Surface((50, 50))
        cuadro_azul.fill(AZUL)
        rect_azul = cuadro_azul.get_rect()
        rect_azul.top = 20 
        rect_azul.left = 20 

        cuadro_amarillo = pygame.Surface((100, 100))
        cuadro_amarillo.fill(AMARILLO)
        rect_amarillo = cuadro_amarillo.get_rect()
        rect_amarillo.top = 300 
        rect_amarillo.left = 300 
       
        self.screen.blit(cuadro_azul, (20, 20))
        self.screen.blit(cuadro_amarillo, (300, 300))

        pygame.display.update()
        
        arrastrar_azul = False
        pos_inicial = None
        while True:
            for event in pygame.event.get():
                if event.type == pygame.QUIT:
                    pygame.quit()
                    sys.exit(0)

                if event.type == pygame.MOUSEBUTTONDOWN:
                    #(x, y)
                    pos = pygame.mouse.get_pos()
                    
                    colisiona_amarillo = rect_amarillo.contains(rect_azul)
                    if arrastrar_azul and colisiona_amarillo:
                        #dibujar el azul dentro del amarillo
                        self.screen.blit(cuadro_amarillo, (300, 300))
                        self.screen.blit(cuadro_azul, pos)
                        pygame.display.update()
                        arrastrar_azul = False
                        continue

                    if rect_azul.collidepoint(pos):
                        arrastrar_azul = True
                        pos_inicial = pos
                        self.screen.blit(self.fondo, (20, 20), rect_azul)
                    else:
                        arrastrar_azul = False

                if event.type == pygame.MOUSEMOTION:
                    if arrastrar_azul:

                        pos = pygame.mouse.get_pos()
                        colisiona_amarillo = rect_amarillo.contains(rect_azul)

                        if pos_inicial and not colisiona_amarillo:
                            #la que hace el truco
                            self.screen.blit(self.fondo, pos_inicial, rect_azul)
                        self.screen.blit(cuadro_amarillo, (300, 300))

                        rect_azul.left, rect_azul.top = pos
                        self.screen.blit(cuadro_azul, rect_azul)
                        pos_inicial = pos
                        pygame.display.update()



if __name__ == "__main__":
    Main().run()