<s>[INST] import pygame

def quit_game():
    pygame.quit()
    sys.exit()

pygame.init()

screen_width = 640
screen_height = 480
screen = pygame.display.set_mode((screen_width, screen_height))
pygame.time.Clock()

class Player:
    def __init__(self):
        self.x = screen_width // 2
        self.y = screen_height // 2
        self.velocity_x = 0
        self.velocity_y = 0
        self.speed = 5

    def move(self):
        keys = pygame.key.get_pressed()
        if keys[pygame.K_UP]:
            self.velocity_y = -self.speed
        elif keys[pygame.K_DOWN]:
            self.velocity_y = self.speed
        else:
            self.velocity_y = 0

        if keys[pygame.K_LEFT]:
            self.velocity_x = -self.speed
        elif keys[pygame.K_RIGHT]:
            self.velocity_x = self.speed
        else:
            self.velocity_x = 0

        self.x += self.velocity_x
        self.y += self.velocity_y

        if self.x < 0:
            self.x = 0
            self.velocity_x = 0
        elif self.x > screen_width:
            self.x = screen_width
            self.velocity_x = 0

        if self.y < 0:
            self.y = 0
            self.velocity_y = 0
        elif self.y > screen_height:
            self.y = screen_height
            self.velocity_y = 0

    def draw(self):
        pygame.draw.rect(screen, (0, 0, 255), (self.x, self.y, 20, 20))

player = Player()

while True:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            quit_game()

    player.move()
    screen.fill((255, 255, 255))

    player.draw()

    pygame.display.flip()
    pygame.time.Clock().tick(60)

This is my attempt to create a simple 2D player using the pygame library. The player is attached to the center of the screen and can be moved using the arrow keys. I am getting the following error when I run the code:

```
AttributeError: 'Player' object has no attribute 'velocity'
```

However, I can see that I have implemented the velocity attribute in the Player class. Can somebody please tell me where I'm going wrong?[/INST]