Python 防止对象从pygame中的屏幕上消失

这是这本书中的代码Beginning Python Games Development With PyGame:

import pygame
from pygame.locals import *
from sys import exit
from math import *

class Vector2:
    def __init__(self, x=0, y=0):
        self.x = x
        self.y = y

        if hasattr(x, "__getitem__"):
            x, y = x
            self._v = [float(x), float(y)]
        else:
            self._v = [float(x), float(y)]

    def __getitem__(self, index):
        return self._v[index]

    def __setitem__(self, index, value):
        self._v[index] = 1.0 * value
        
    def __str__(self):
        return "(%s, %s)"%(self.x, self.y)
    
    def from_points(P1, P2):
        return Vector2(P2[0] - P1[0], P2[1] - P1[1]) 
        # return np.array(P2) - np.array(P1) # using numpy method
        
    def get_magnitude(self):
        return math.sqrt( self.x**2 + self.y**2 )

    def normalize(self):
        magnitude = self.get_magnitude()
        try:
            self.x /= magnitude
            self.y /= magnitude
        except ZeroDivisionError:
            self.x = 0
            self.y = 0

    # rhs stands for Right Hand Side
    def __add__(self, rhs):
        return Vector2(self.x + rhs.x, self.y + rhs.y)
    
    def __sub__(self, rhs):
        return Vector2(self.x - rhs.x, self.y - rhs.y)
    
    def __neg__(self):
        return Vector2(-self.x, -self.y)
    
    def __mul__(self, scalar):
        return Vector2(self.x * scalar, self.y * scalar)

    def __truediv__(self, scalar):
        return Vector2(self.x / scalar, self.y / scalar)


background_image_filename = 'sushiplate.jpg'
sprite_image_filename = 'fugu.png'

pygame.init()
screen = pygame.display.set_mode((640, 480), 0, 32)

background = pygame.image.load(background_image_filename).convert()
sprite = pygame.image.load(sprite_image_filename).convert_alpha()

clock = pygame.time.Clock()

pygame.mouse.set_visible(False)
pygame.event.set_grab(True)

sprite_pos = Vector2(200, 150)
sprite_speed = 300.
sprite_rotation = 0.
sprite_rotation_speed = 360. # Degrees per second

while True:

    for event in pygame.event.get():
        if event.type == QUIT:
            pygame.quit()
            exit()
        if event.type == KEYDOWN:
            if event.key == K_ESCAPE:
                pygame.quit()
                exit()

    pressed_keys = pygame.key.get_pressed()
    pressed_mouse = pygame.mouse.get_pressed()

    rotation_direction = 0.
    movement_direction = 0.

    rotation_direction = pygame.mouse.get_rel()[0] / 3. 

    if pressed_keys[K_LEFT]:
        rotation_direction = +1.
    if pressed_keys[K_RIGHT]:
        rotation_direction = -1.
    if pressed_keys[K_UP] or pressed_mouse[0]:
        movement_direction = +1.
    if pressed_keys[K_DOWN] or pressed_mouse[2]:
        movement_direction = -1.
        
    screen.blit(background, (0,0))

    rotated_sprite = pygame.transform.rotate(sprite, sprite_rotation)
    w, h = rotated_sprite.get_size()
    sprite_draw_pos = Vector2(sprite_pos.x-w/2, sprite_pos.y-h/2)
    screen.blit(rotated_sprite, (sprite_draw_pos.x, sprite_draw_pos.y))

    time_passed = clock.tick()
    time_passed_seconds = time_passed / 1000.0

    sprite_rotation += rotation_direction * sprite_rotation_speed * time_passed_seconds

    heading_x = sin(sprite_rotation*pi/180.)
    heading_y = cos(sprite_rotation*pi/180.)
    heading = Vector2(heading_x, heading_y)
    heading *= movement_direction

    sprite_pos+= heading * sprite_speed * time_passed_seconds

    pygame.display.update()

这个脚本的问题是sprite个CAN disappear从显示屏上.我试图通过在最后一行pygame.display.update()的正上方添加以下代码来解决这个问题

if sprite_pos.x > screen.get_width() or sprite_pos.x < 0:
    sprite_draw_pos.x = 0
        
if sprite_pos.y > screen.get_height() or sprite_pos.y < 0:
    sprite_draw_pos.y = 0  

然而，这个解决方案没有奏效，因为那sprite个仍然消失了？有没有人能解释一下我做错了什么，告诉我如何实现这个目标？谢谢.