The loop() function causes draw() to execute repeatedly. If noLoop is called in setup() the draw() is only executed once. In this example click the mouse to execute loop(), which will cause the draw() to execute repeatedly.

y = 100

def setup():
    The statements in the setup() function
    run once when the program begins.
    size(640, 360)    # Size should be the first statement
    stroke(255)       # Set stroke color to white
    y = height * 0.5

def draw():
    The statements in draw() are run until the
    program is stopped. Each statement is run in
    sequence and after the last line is read, the first
    line is run again.
    global y
    background(0)     # Set the background to black
    line(0, y, width, y)
    y = y - 1
    if y < 0:
        y = height

def mousePressed():