Python - Pygame - put two pixels using mouse click

import pygame

# === CONSTANTS ===

BLACK = (  0,   0,   0)
WHITE = (255, 255, 255)

PIXEL_SIZE = 10

# === CLASSES ===

# empty

# === FUNCTIONS ===

# empty

# === DATA ===

colors = {
    '#': BLACK,
    ' ': WHITE,
}

data = [
    "      ########   ########      ",
    " ##   #      #   #      #   ## ",
    "#  #  #      #   #      #  #  #",
    "#   # #      #   #      # #   #",
    "     ##      #####      ##     ",
    "      #      #####      #      ",
    "      #      #   #      #      ",
    "      #      #   #      #      ",
    "      #      #   #      #      ",
    "      #      #   #      #      ",
    "      #      #   #      #      ",
    "      #      #   #      #      ",
    "      ########   ########      ",
]

# === MAIN ===

# --- init ---

pygame.init()

screen = pygame.display.set_mode([400,400])
pygame.display.set_caption('okulary')

# --- draws ---

screen.fill(WHITE)

# create only once
rect = pygame.Rect(0, 0, PIXEL_SIZE, PIXEL_SIZE)

for y, row in enumerate(data):
    for x, char in enumerate(row):
        rect.x = x*PIXEL_SIZE
        rect.y = y*PIXEL_SIZE
        screen.fill(colors[char], rect=rect)

pygame.display.update()

# --- mainloop ---

running = True

while running:

    for event in pygame.event.get():
        if event.type == pygame.QUIT:
           running = False

        elif event.type == pygame.MOUSEBUTTONDOWN:

            if event.button == 1: # left button

                # Python 2: rounding to integer using /
                # Python 3: rounding to integer using //
                col = event.pos[0] // PIXEL_SIZE
                row = event.pos[1] // PIXEL_SIZE
                # first pixel
                rect.x = col*PIXEL_SIZE
                rect.y = row*PIXEL_SIZE
                screen.fill(BLACK, rect=rect) # BLACK
                # pixel on the left
                rect.x -= PIXEL_SIZE
                screen.fill(BLACK, rect=rect) # BLACK

                # send on monitor
                pygame.display.update()

            elif event.button == 3: # right button

                # Python 2: rounding to integer using /
                # Python 3: rounding to integer using //
                col = event.pos[0] // PIXEL_SIZE
                row = event.pos[1] // PIXEL_SIZE
                # first pixel
                rect.x = col*PIXEL_SIZE
                rect.y = row*PIXEL_SIZE
                screen.fill(WHITE, rect=rect) # WHITE
                # pixel on the left
                rect.x -= PIXEL_SIZE
                screen.fill(WHITE, rect=rect) # WHITE

                # send on monitor
                pygame.display.update()

# --- the end ---

pygame.quit()