Pygame string-based level draws once then disappears

Question

I'm using Python 2.7. I'm having trouble with redrawing a string-based level. It draws fine for the first frame, but after that, it just doesn't seem to draw anymore. I've been trying to come up with a fix for a while now. Here is the entire source code:

import pygame
from pygame.locals import *

xScan = 0
yScan = 0
level = "WWWWWWWW" \
        "WSSSSSSW" \
        "WSSSSSSW" \
        "WSSSSSSW" \
        "WSSSSSSW" \
        "WWWWWWWW"


class Player():
    def __init__(self, color, x, y, width, height, outline_size):
        self.color = color
        self.x = x
        self.y = y
        self.width = width
        self.height = height
        self.outline_size = outline_size

    def draw(self):
        pygame.draw.rect(screen, self.color, [self.x * 80, self.y * 80, self.width * 80, self.height * 80], self.outline_size)


def draw_level():
    for tile in level:
        global xScan, yScan
        if tile == "W":
            pygame.draw.rect(screen, [0, 0, 0], [xScan * 80, yScan * 80, 80, 80], 0)
        elif tile == "S":
            pygame.draw.rect(screen, [120, 120, 120], [xScan * 80, yScan * 80, 80, 80], 1)

        if xScan >= 0 and xScan < 7:
            xScan += 1
        elif xScan == 7:
            xScan = 0
            yScan += 1
        elif yScan == 5 and xScan == 7:
            xScan = 0
            yScan = 0


def update():
    screen.fill([255, 255, 255])
    draw_level()
    player.draw()
    pygame.display.flip()

pygame.init()
screen = pygame.display.set_mode([640, 480])
pygame.display.set_caption("Squares n' Tiles!")
player = Player([0, 100, 255], 1, 1, 1, 1, 0)

running = True
while running:
    clock = pygame.time.Clock()
    clock.tick(30)
    for event in pygame.event.get():
        if event.type == pygame.KEYDOWN:
            if event.key == K_ESCAPE:
                running = False
            if event.key == K_UP and player.y * 80 >= 160:
                player.y -= 1
            if event.key == K_DOWN and player.y * 80 < 320:
                player.y += 1
            if event.key == K_LEFT and player.x * 80 >= 160:
                player.x -= 1
            if event.key == K_RIGHT and player.x * 80 < 480:
                player.x += 1
            if event.key == K_c or event.key == K_r:
                player.x = 1
                player.y = 1
        if event.type == pygame.QUIT:
            running = False
    update()

pygame.quit()

I think the problem lies within this portion of the code:

def draw_level():
    for tile in level:
        global xScan, yScan
        if tile == "W":
            pygame.draw.rect(screen, [0, 0, 0], [xScan * 80, yScan * 80, 80, 80], 0)
        elif tile == "S":
            pygame.draw.rect(screen, [120, 120, 120], [xScan * 80, yScan * 80, 80, 80], 1)

        if xScan >= 0 and xScan < 7:
            xScan += 1
        elif xScan == 7:
            xScan = 0
            yScan += 1
        elif yScan == 5 and xScan == 7:
            xScan = 0
            yScan = 0

I've tried messing with the numbers in the latter half of the previous portion. Here is one variation that mostly fixes the problem (though its format isn't very consistent):

if xScan >= 7:
    xScan = 0
    yScan += 1
elif yScan == 6:
    xScan = 1
    yScan = 0
else:
    xScan += 1

The only problem with the above code is that the top left tile doesn't draw. How could I vary up my code to make it work properly? On every frame, I want the screen to be filled with white, the level re-drawn, the player re-drawn, and the screen flipped. Thanks in advance for any help. I would also appreciate advice on my code's structure.

Answer 1

The problem is that this block isn't executed:

    elif yScan == 5 and xScan == 7:
        xScan = 0
        yScan = 0

This is because you're looping over each character in the level string, so the max value reached is xScan = 6, yScan = 5.

Instead just initialise xScan and yScan when entering the draw level function

def draw_level():
    xScan = 0 # no need for these to be global
    yScan = 0
    for tile in level:
        if tile == "W":
            pygame.draw.rect(screen, [0, 0, 0], [xScan * 80, yScan * 80, 80, 80], 0)
        elif tile == "S":
            pygame.draw.rect(screen, [120, 120, 120], [xScan * 80, yScan * 80, 80, 80], 1)

        if xScan >= 0 and xScan < 7:
            xScan += 1
        elif xScan == 7:
            xScan = 0
            yScan += 1

I would suggest storing your level as several lines of characters instead of one long string:

level = "WWWWWWWW\n" \
        "WSSSSSSW\n" \
        "WSSSSSSW\n" \
        "WSSSSSSW\n" \
        "WSSSSSSW\n" \
        "WWWWWWWW\n" # note the new line characters (\n)

Then you can iterate over the map like so:

def draw_level():
    for yScan, row in enumerate(level.split('\n')):
        for xScan, tile in enumerate(row):
            if tile == "W":
                pygame.draw.rect(screen, [0, 0, 0], [xScan * 80, yScan * 80, 80, 80], 0)
            elif tile == "S":
                pygame.draw.rect(screen, [120, 120, 120], [xScan * 80, yScan * 80, 80, 80], 1)

enumerate takes a list (or iterable) and returns pairs of (index, item) for each item in the list, for example:

>>> a = 'test'
>>> for i, c in enumerate(a):
...     print('i is {}, c is {}'.format(i, c))
...
i is 0, c is t
i is 1, c is e
i is 2, c is s
i is 3, c is t