PyGame code modification and adding following camera

# http://stackoverflow.com/questions/34566528/how-do-you-get-the-camera-to-follow-the-player-scrolling

import pygame

# --- CONSTANS --- UPPERCASE names

DISPLAY_WIDTH = 1200
DISPLAY_HEIGHT = 800

RED   = (255,  0,  0)
GREEN = (  0,255,  0)

BLACK = (  0,  0,  0)

BLOCK_WALL = "P"
BLOCK_PLAYER = "$"
#BLOCK_OTHER = ""

PLAYER_SIZE = 50

# --- CLASSES --- CamelCase names

# --- FUNCTIONS --- lower_case names

def draw_block(screen, rect, color=RED):
    pygame.draw.rect(screen, color, rect)

# --- MAIN ---

# --- variables --- lower_case names

# two-dimensional list
level =  [
    "PPPPPPPPPPPP", # or sublist [ "P","P","P","P","P","P","P","P","P","P","P" ],
    "P          P", # or sublist [ "P"," "," "," "," "," "," "," "," "," ","P" ],
    "P        P P", # or sublist [ "P"," "," "," "," "," "," "," "," ","P","P" ],
    "P          P", # or sublist [ "P"," "," "," "," "," "," "," "," "," ","P" ],
    "P     P    P", # or sublist [ "P"," "," "," "," "," ","P"," "," "," ","P" ],
    "P   $ P    P", # or sublist [ "P"," "," "," ","$"," ","P"," "," "," ","P" ],
    "P          P", # or sublist [ "P"," "," "," "," "," "," "," "," "," ","P" ],
    "P          P", # or sublist [ "P"," "," "," "," "," "," "," "," "," ","P" ],
    "P   P      P", # or sublist [ "P"," "," "," ","P"," "," "," "," "," ","P" ],
    "PPPPPPPPPPPP", # or sublist [ "P","P","P","P","P","P","P","P","P","P","P" ],
]

# --- init ----

pygame.init()

game_display = pygame.display.set_mode((DISPLAY_WIDTH, DISPLAY_HEIGHT))
game_display_rect = game_display.get_rect()

pygame.display.set_caption("My Game")

# --- player ---

# TODO: find player position on map (`level`)

player_rect = pygame.Rect(0, 0, PLAYER_SIZE, PLAYER_SIZE)
player_rect.center = game_display_rect.center

player_movement = 5
player_change_y = 0
player_change_x = 0

# --- walls ---

# CALCULATING THE SIZE OF THE BLOCKS SO THEY FIT W/ A VARIETY OF RESOLUTIONS

# scale > 1 to make level bigger then screen size - to use following camera
scale = 2

block_width = DISPLAY_WIDTH/len(level[0]) * scale
block_height = DISPLAY_HEIGHT/len(level) * scale

walls_list = []

for y, row in enumerate(level):
    for x, element in enumerate(row):
        if element == BLOCK_WALL:
            rect = pygame.Rect(x*block_width, y*block_height, block_width, block_height)
            walls_list.append( rect )

print('walls count:', len(walls_list))

# --- offset ---

offset_x = 0
offset_y = 0

max_offset_x = (len(level[0]) * block_width) - game_display_rect.width
max_offset_y = (len(level) * block_height) - game_display_rect.height

print('max_offset_x:', max_offset_x)

# --- mainloop ---

clock = pygame.time.Clock()

game_exit = False

while not game_exit:

    # --- events handling ---

    for event in pygame.event.get():

        # --- global events ---

        if event.type == pygame.QUIT:
            game_exit = True

        if event.type == pygame.KEYDOWN:
            if event.key == pygame.K_ESCAPE:
                game_exit = True

        # --- player events ---

        if event.type == pygame.KEYDOWN:
            if event.key == pygame.K_UP:
                player_change_y -= player_movement

            if event.key == pygame.K_DOWN:
                player_change_y += player_movement

            if event.key == pygame.K_LEFT:
                player_change_x -= player_movement

            if event.key == pygame.K_RIGHT:
                player_change_x += player_movement

        elif event.type == pygame.KEYUP:
            if event.key == pygame.K_UP:
                player_change_y += player_movement

            if event.key == pygame.K_DOWN:
                player_change_y -= player_movement

            if event.key == pygame.K_LEFT:
                player_change_x += player_movement

            if event.key == pygame.K_RIGHT:
                player_change_x -= player_movement


    # --- calculations (without draws) ---

    # move player and check collision

    # left-right collision
    if player_change_x:
        player_rect.x += player_change_x

        for wall_rect in walls_list:

            if player_rect.colliderect(wall_rect):
                if player_change_x < 0: # move left
                    player_rect.left = wall_rect.right
                elif player_change_x > 0: # move right
                    player_rect.right = wall_rect.left

    # up-down collision
    if player_change_y:
        player_rect.y += player_change_y

        for wall_rect in walls_list:

            if player_rect.colliderect(wall_rect):
                if player_change_y < 0: # move up
                    player_rect.top = wall_rect.bottom
                elif player_change_y > 0: # move down
                    player_rect.bottom = wall_rect.top

    # camera offset

    # left side stop
    if player_rect.x > game_display_rect.centerx:
        offset_x = game_display_rect.centerx - player_rect.x
        # right side stop
        if offset_x < -max_offset_x:
            offset_x = -max_offset_x

    # top side stop
    if player_rect.y > game_display_rect.centery:
        offset_y = game_display_rect.centery - player_rect.y
        # bottom side stop
        if offset_y < -max_offset_y:
            offset_y = -max_offset_y

    # --- draws (without calculations )---

    # clear screen
    game_display.fill(BLACK)

    # draw walls with offset
    for wall_rect in walls_list:
        draw_block(game_display, wall_rect.move(offset_x, offset_y))

    # draw player with offset
    pygame.draw.rect(game_display, GREEN, player_rect.move(offset_x, offset_y))

    clock.tick(120) # most monitors refresh screen with 60Hz - it means 60 FPS

    # send to monitor
    pygame.display.update()

# --- the end ---

pygame.quit()
quit()