Scrolling avec pygame

import pygame
class Level():
    def __init__(self) :
        self.move_speed = 5
        self.tile_width = 24
        self.tile_height = 16
        self.nb_tilesx_screen = 15
        self.nb_tilesy_screen = 13
        self.nb_tilesx_world = 75
        self.nb_tilesy_world = 17
        self.screen_width = self.tile_width*self.nb_tilesx_screen
        self.screen_height = self.tile_height*self.nb_tilesy_screen
        self.xscroll = 0
        self.yscroll = (self.nb_tilesy_world-self.nb_tilesy_screen)*self.tile_height
        self.level = []
        self.tiles = pygame.image.load('pictures\\tiles.png')
        self.tileset = {'0' : [self.tiles.subsurface((0,0,24,16)),'empty','sky'],
                        '1' : [self.tiles.subsurface((24,0,24,16)),'full','treat'],
                        '2' : [self.tiles.subsurface((48,0,24,16)),'full','brick'],
                        '3' : [self.tiles.subsurface((72,0,24,16)),'full','tubetop1'],
                        '4' : [self.tiles.subsurface((96,0,24,16)),'full','tubetop2'],
                        '5' : [self.tiles.subsurface((120,0,24,16)),'full','tubebody1'],
                        '6' : [self.tiles.subsurface((144,0,24,16)),'full','tubebody2'],
                        '7' : [self.tiles.subsurface((168,0,24,16)),'full','block']}


    def load_map(self,level):

        with open(level,'r') as level :
            m = []
            item = ''
            for line in level :
                line_level = []
                for col in line :
                    if col != '\n' and col != ' ' :
                        item += col
                    else :
                        line_level.append(item)
                        item = ''
                m.append(line_level)
            self.level = m



    def display_map(self,screen):

        xmin = (self.xscroll/self.tile_width)-1
        xmax = (self.xscroll+self.screen_width)/self.tile_width
        ymin = (self.yscroll/self.tile_height)-1
        ymax = (self.yscroll+self.screen_height)/self.tile_height

        for i in range(ymin,ymax) :
            for j in range(xmin,xmax) :
                x = j*self.tile_width-self.xscroll
                y = i*self.tile_height-self.yscroll
                item = self.level[i][j]
                screen.blit(self.tileset[item][0],(x,y))


        pygame.display.flip()


    def screen_size(self):
        return (self.tile_width*self.nb_tilesx_screen,self.tile_height*self.nb_tilesy_screen)

    def move(self,direction):

        xscroll = self.xscroll
        yscroll = self.yscroll
        if direction == 'right' :
            xscroll += self.move_speed
            if xscroll > self.nb_tilesx_world*self.tile_width-self.screen_width-1 :
                self.xscroll = self.nb_tilesx_world*self.tile_width-self.screen_width-1
            else :
                self.xscroll = xscroll

        elif direction == 'left' :
            xscroll -= self.move_speed
            if xscroll < 0:
                self.xscroll = 0
            else :
                self.xscroll = xscroll

        elif direction == 'up' :
            yscroll -= self.move_speed
            if yscroll < 0 :
                self.yscroll = 0
            else :
                self.yscroll = yscroll

        elif direction == 'down' :
            yscroll += self.move_speed
            if yscroll > self.nb_tilesy_world*self.tile_height-self.screen_height-1 :
                self.yscroll = self.nb_tilesy_world*self.tile_height-self.screen_height-1
            else :
                self.yscroll = yscroll

import pygame
from level import *

pygame.init()
horloge = pygame.time.Clock()
level = Level()
level.load_map('level.txt')
screen = pygame.display.set_mode(level.screen_size())
main = 1
direction = ''
while main :
    horloge.tick(30)
    for event in pygame.event.get() :
        if event.type == pygame.QUIT :
            main = 0
        if event.type == pygame.KEYDOWN :
            if event.key == pygame.K_RIGHT :
                direction = 'right'
            elif event.key == pygame.K_LEFT :
                direction = 'left'
            elif event.key == pygame.K_UP :
                direction = 'up'
            elif event.key == pygame.K_DOWN :
                direction = 'down'

        if event.type == pygame.KEYUP :
            if event.key == pygame.K_RIGHT and direction == 'right' :
                direction = ''
            elif event.key == pygame.K_LEFT and direction == 'left' :
                direction == ''
            elif event.key == pygame.K_UP and direction == 'up' :
                direction == ''
            elif event.key == pygame.K_DOWN and direction == 'down' :
                direction == ''

    level.move(direction)
    level.display_map(screen)
pygame.quit()

`