# Importing the Pygame library for building games and multimedia applications.
import pygame
# Importing the sys module to exit the program.
import sys

# Initializing all Pygame modules.
pygame.init()
# Creating a window surface with a size of 400x300 pixels.
screen = pygame.display.set_mode((400, 300))
# Creating a background surface with the same size as the window.
background = pygame.Surface((400, 300))

# Creating a surface with a size of 10x10 pixels.
surface = pygame.Surface((10, 10))
# Filling the surface with a green color.
surface.fill((0,255,0))

# Setting the initial position for the surface to be displayed.
pos = [175, 125]

# Main game loop that runs indefinitely until the program is exited.
while True:
    # Getting all the events that have occurred.
    events = pygame.event.get()
    # Iterating through each event.
    for event in events:
        # If the event is the user closing the window...
        if event.type == pygame.QUIT:
            # ...quit Pygame and exit the program.
            pygame.quit()
            sys.exit()
        # If the event is a key press...
        if event.type == pygame.KEYDOWN:
            # ...and the pressed key is the right arrow key...
            if event.key == pygame.K_RIGHT:
                # ...move the surface 20 pixels to the right.
                pos[0] += 20

    # Drawing the background on the window at position (0,0).
    screen.blit(background, (0,0))            
    # Drawing the surface on the window at the current position.
    screen.blit(surface, pos)
    # Updating the display to show the changes.
    pygame.display.update()