"""
Animating multiple objects using a list.
Sample Python/Pygame Programs
Simpson College Computer Science
http://simpson.edu/computer-science/

Explanation video: http://youtu.be/Gkhz3FuhGoI
"""

# Import a library of functions called 'pygame'
import pygame
import random

# Initialize the game engine
pygame.init()

BLACK = [0, 0, 0]
WHITE = [255, 255, 255]

# Set the height and width of the screen
SIZE = [400, 400]

screen = pygame.display.set_mode(SIZE)
pygame.display.set_caption("Snow Animation")

# Create an empty array
snow_list = []

# Loop 50 times and add a snow flake in a random x,y position
for i in range(50):
x = random.randrange(0, 400)
y = random.randrange(0, 400)
snow_list.append([x, y])

clock = pygame.time.Clock()

# Loop until the user clicks the close button.
done = False
while not done:

for event in pygame.event.get():   # User did something
if event.type == pygame.QUIT:  # If user clicked close
done = True   # Flag that we are done so we exit this loop

# Set the screen background
screen.fill(BLACK)

# Process each snow flake in the list
for i in range(len(snow_list)):

# Draw the snow flake
pygame.draw.circle(screen, WHITE, snow_list[i], 2)

# Move the snow flake down one pixel
snow_list[i][1] += 1

# If the snow flake has moved off the bottom of the screen
if snow_list[i][1] > 400:
# Reset it just above the top
y = random.randrange(-50, -10)
snow_list[i][1] = y
# Give it a new x position
x = random.randrange(0, 400)
snow_list[i][0] = x

# Go ahead and update the screen with what we've drawn.
pygame.display.flip()
clock.tick(20)

# Be IDLE friendly. If you forget this line, the program will 'hang'
# on exit.
pygame.quit()

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