import pygame
import sys
from pygame.locals import *


pygame.init()

size = width,height = 640,480
bg = (0,0,0)

clock = pygame.time.Clock()
screen = pygame.display.set_mode(size)
pygame.display.set_caption("tang Demo")

turtle = pygame.image.load("turtle.png").convert_alpha()
background = pygame.image.load("background.jpg").convert()
position = turtle.get_rect()
position.center = width //2 ,height //2

def blit_alpha(target,source,location,opacity):
    x = location[0]
    y = location[1]
    temp = pygame.Surface((source.get_width(),source.get_height())).convert()
    temp.blit(target,(-x,-y))
    temp.blit(source,(0,0))
    temp.set_alpha(opacity)
    target.blit(temp,location)

while True:
    for event in pygame.event.get():
        if event.type == QUIT:
            sys.exit()
    screen.blit(background,(0,0))
    blit_alpha(screen,turtle,position,200)

    pygame.display.flip()
    clock.tick(30)