Help with "while False:" loop

cesco

Disclaimer: Whilst my experiment implements PyGaze functions, I felt that this was more of an OpenSesame issue. Hence, I posted my question in the OpenSesame sub-forum.

Background: I am programming a gaze-contingent eye-tracking design, starting with a fixation dot. Originally this is a sketchpad, but for the current status I found it easier to use the canvas functions. I have some Python experience (I created an ssVEP/EEG experiment with PsychoPy) and used the for-loop in Matlab/PsychToolbox, but the while-loop is reasonably new to me.

What I want it to do: A fixation dot is presented, and an area of interest is drawn around it, let's call this fixation-area. If the participant does not fixate the fixation-area, nothing will happen until it times out. Once the gaze is directed at the fixation-area, a timer will start. After 500 ms the next screen will be presented.

Please note that the following coordinates are not final: I currently present the experiment in a small window to keep an eye on the debug-window. And I noticed that PyGaze does not use uniform coordinates, so I unticked this. The fixation dot and AOI on this screen are in the centre.

Prepare phase:

# Set up constants. Note that I am drawing a visual representation of my AOI. Coordinates are not final. 

my_canvas = canvas()
my_canvas.fixdot()
my_canvas.circle(350, 195, 100, fill=False, color='red')

# Set the Area around the fixtion dot (type, pos, size)
FixAOI = AOI('circle', (350,195), 100)
FixDuration = 495
timeout = 10000

Run phase:

# Show the screen. Eventually I want to use sketchpad items instead of canvases.
my_canvas.show()

# Get the current time and print this for debugging purposes.
t0 = self.time()
print('T0 %f' % t0)

# Get and print the x,y tuple of the gaze position. Start the timer.
GazePos = eyetracker.sample()
print('GazePos = ', GazePos)
var.current_time = self.time() - t0

# As long as the gaze position is not within the AOI, wait until it is or time out. This calls on the imported AOI plugin. Sleep command set for now to avoid drowning the debug window 
while False:
    FixAOI.contains(GazePos)
    var.current_time = self.time() - t0
    if var.current_time < timeout:
        print('Duration if not hit =', var.current_time)
        clock.sleep(50)
    elif var.current_time >= timeout:
        print('timed out')
        break
    else:
        print('FML')
        break

# When the gaze position IS within the AOI, count the timer until the given fixatio duration is reached.    
while True:
    FixAOI.contains(GazePos)
    var.current_time = self.time() - t0
    if var.current_time < FixDuration:
        #t1 >= FixDuration
        print(u'T1 during hit', var.current_time)
        clock.sleep(10)
    elif var.current_time >= FixDuration:
        print('Fixated!')
        break
    else:
        print('hit but FML')
        break

What it does: it skips the False part altogether, even though GazePos is printed as outside of the fixation-area. The timer seems to work okay and OpenSesame succesfully presents the next screen, but it appears to just see FixAOI.contains(GazePos) as True.

It is probably just a rookie error in creating the loop. Any help would be greatly appreciated!

sebastiaan

Hi,

You're misunderstanding the semantics of a while loop!

This:

while True:
    something()

Means: enter an infinite loop that runs something() over and over again. It does not mean: run the loop while something() is True.

Analogously, this:

while False:
    something()

Means: never enter the loop (because the criterion is already False on the first iteration)—in other words, it does nothing. It does not mean: run the loop while something() is False. This does:

while not something():
    pass

However, I think it's best to take a step back and familiarize yourself with basic Python syntax. That won't take long, and will avoid you from getting stuck every step of the way.

Cheers!
Sebastiaan

cesco

Thanks @sebastiaan, that really cleared things up! I made it work with the following script. It needs some tweaking as it is ugly, but moreover, even with the clock.sleep() function the execution seems to carry on after the onset of the next screen (although maybe that was just when I printed debugging data, but I don't want to chance it).

my_canvas.show()

# As long as the gaze position is not within the AOI, wait until it is or time out. This calls on the imported AOI plugin. Sleep command set for now to avoid drowning the debug window 

# Start by declaring that the AOI is not fixated (HitAOI) for enough time (is_fixation)
HitAOI = False
is_fixation = False

# So it enters this loop, which keeps updating the Gaze position and starts the timer, but resets it if gaze is averted from AOI.
while HitAOI is False or is_fixation is False: 
    GazePos = eyetracker.sample()
    print('GazePos = ', GazePos)
    t0 = clock.time()
    HitAOI = False
    clock.sleep(5)
    # While a sample is detected in the AOI but not for 495 ms yet, start counting until 495:   
    while FixAOI.contains(GazePos) and is_fixation is False:
        HitAOI = True
        GazePos = eyetracker.sample()
        t1 = clock.time() - t0
        clock.sleep(5)
        # Gaze sample within AOI for 495 ms, exit the loop and move on! 
        if t1 >= FixDuration:
            is_fixation = True
            clock.sleep(5)
        # But any less than 495 ms, keep counting
        else:
            is_fixation = False
            clock.sleep(5)

Wonderful, thanks again!