Flickering canvases in OpenSesame

import random
from openexp.keyboard import keyboard

# Inter-stimulus interval
ITI = 20

# The number of cycles
n_cycles = 20

# Determines whether the canvas list is presented in random order
# or sequentially
random_order = False

# Set-up a keyboard object with a timeout equal to the interval
# between the two stimuli
my_keyboard = keyboard(exp, timeout=1)

# Copy a number of canvases from sketchpads and put them in a list
canvas_list = [
    self.copy_sketchpad('sketchpad_1'),
    self.copy_sketchpad('sketchpad_2'),
    self.copy_sketchpad('sketchpad_3')
    ]

# Start the response interval
start_timestamp = self.time()

# We are going to keep a list of responses
resp_list = []

# Go into a loop where the two canvases are shown in alternation
for i in range(n_cycles):

    # Show the first canvas
    if random_order:
        my_canvas = random.choice(canvas_list)
    else:
        my_canvas = canvas_list[i % len(canvas_list)]

    # Show the canvas
    t1 = my_canvas.show()

    # Use the get_key() function to sleep and monitor for keypresses
    # at the same time.
    while self.time() - t1 < ITI:
        resp, t2 = my_keyboard.get_key()
        if resp != None:
            # If a key was pressed, add it to the list of responses.
            resp_list.append( (resp, t2) )

# If the response_list is empty, no response was given
if len(resp_list) == 0:
    response = 'timeout'
    response_time = 0

# Otherwise get the first response from the list
else:
    resp, resp_timestamp = resp_list[0]
    response = resp
    response_time = start_timestamp - resp_timestamp

# Set the response variables!
exp.set('response', resp)
exp.set('response_time', response_time)

# To maintain feedback variables, see
# <http://osdoc.cogsci.nl/usage/giving-feedback-to-participants/#inline-script>