Loop

The loop() function causes draw() to execute continuously. 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() the execute continuously.

from p5 import *

y = 0

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

    global y
    y = height * 0.5

# 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) # Set the background to black
    global y
    y = y - 1
    if y < 0:
        y = height

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

def mouse_pressed():
    loop()

if __name__ == '__main__':
    run()