The noLoop() function causes draw() to only execute once. Without calling noLoop(), the code inside draw() is run continually.
y = 180
def setup():
"""
The statements in the setup() function
execute once when the program begins
"""
size(640, 360) # Size should be the first statement
stroke(255) # Set line drawing color to white
noLoop()
def draw():
"""
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.
"""
global y
background(0) # Set the background to black
y = y - 1
if (y < 0):
y = height
line(0, y, width, y)