问在Python中，类似字符的列表映射是如何工作的？

EN

import pygame
import time
import sys

# Window size
WINDOW_WIDTH=400
WINDOW_HEIGHT=400

pygame.init()
WINDOW  = pygame.display.set_mode( ( WINDOW_WIDTH, WINDOW_HEIGHT ), pygame.HWSURFACE|pygame.DOUBLEBUF|pygame.RESIZABLE )
pygame.display.set_caption("Map Loader")

clock = pygame.time.Clock()

TILE_SIZE = 32
wall_image  = pygame.image.load('brick_32.png')
grass_image = pygame.image.load('grass_32.png')

level = [
    "PPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPP",
    "P                                          P",
    "P                                          P",
    "P                                          P",
    "P                    PPPPPPPPPPP           P",
    "P                                          P",
    "P                                          P",
    "P                                          P",
    "P    PPPPPPPP                              P",
    "P                                          P",
    "P                          PPPPPPP         P",
    "P                 PPPPPP                   P",
    "P                                          P",
    "P         PPPPPPP                          P",
    "P                                          P",
    "P                     PPPPPP               P",
    "P                                          P",
    "P   PPPPPPPPPPP                            P",
    "P                                          P",
    "P                 PPPPPPPPPPP              P",
    "P                                          P",
    "P                                          P",
    "P                                          P",
    "P                                          P",
    "PPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPP",]


def drawMap( screen, level ):
    level_width = len(level[0])  # width for first row
    level_height = len(level)    # number of rows

    for y in range( level_height ):
        for x in range ( level_width ):
            if ( level[y][x] == 'P' ):
                screen.blit( wall_image, ( x*TILE_SIZE, y*TILE_SIZE) )
            elif ( level[y][x] == ' ' ):
                screen.blit( grass_image, (x*TILE_SIZE, y*TILE_SIZE) )

def drawGameWindow( screen ):
    global WINDOW_WIDTH, WINDOW_HEIGHT
    drawMap( screen, level )
    pygame.display.update()
    pygame.display.flip()


clock = pygame.time.Clock()
done = False
while not done:

    for event in pygame.event.get():
        if ( event.type == pygame.QUIT ):
            done = True
        elif ( event.type == pygame.VIDEORESIZE ):
            WINDOW_WIDTH  = event.w
            WINDOW_HEIGHT = event.h
            WINDOW  = pygame.display.set_mode( ( WINDOW_WIDTH, WINDOW_HEIGHT ), pygame.HWSURFACE|pygame.DOUBLEBUF|pygame.RESIZABLE )

    # Movement keys
    keys = pygame.key.get_pressed()
    if ( keys[pygame.K_UP] ):
        print("up")
    elif ( keys[pygame.K_DOWN] ):
        print("down")
    elif ( keys[pygame.K_LEFT] ):
        print("left")
    elif ( keys[pygame.K_RIGHT] ):
        print("right")

    drawGameWindow( WINDOW )
    clock.tick_busy_loop( 60 )


pygame.quit()