Setup and Draw

The code inside the draw() function runs continuously from top to bottom until the program is stopped.

from p5 import *

# The statement in setup() function
# ececute once when the program begins
def setup():
        size(640, 360) # size must be the first statement
        stroke(255) # Set line drawing color to white
        y = 0

# The statements in draw() are executed until the
# program is stopped. Each statement is executed in
# sequence and after the last line is read, the first
# line is executed again.
def draw():
        background(0) # Clear the screen with a black background
        y = y - 1
        if y < 0:
                y = height

        line((0, y), (width, y))


if __name__ == '__main__':
    run()