Receiving KeyError while trying to terminate a loop

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1

Hi,

I’m using PsychoPy v2024.2.4 and what I am trying to do is to end my practice loop after 5 trials. This bit of code;

’’if practice.thisN == 4:
practice.finished = True’’

in the “end routine” tab used to work perfectly but suddenly it started to terminate the whole experiment instead of skipping to the next routine while giving a “KeyError.”
(raise KeyError(key) from err
KeyError: 5)

I tried adding this part to the “begin routine” tab but it didn’t help.

’’if practice.thisN == 4:
continueRoutine = False’’

I also tried other solutions suggested here (e.g., skipping-a-routine-after-a-number-of-loops, psychopy-ends-experiment-when-loop-is-terminated) but couldn’t make it. Any help would be so much appreciated. Thanks!

Screenshot 2025-10-18 at 14.27.36
Screenshot 2025-10-18 at 14.27.362446×504 25.7 KB

2

Hi There,

I suspect this is because the key is read as a string rather than and integer. Could you try adjusting your code from:

if practice.thisN == 4:

To

if practice.thisN == '4':

Hopefully that does the trick!

Becca

3

.thisN is usually an integer not text. However, I feel like I have come across this issue. Are you using any other code related to the loop, such as rewind trials?

4

Helloo,

Thank you both! Unfortunately, quotation marks didn’t work.

The task is a dot task where two circles with different numbers of dots are presented on the left and right sides of the screen. I have the following codes to tell PsychoPy to randomly choose images of these circles from randomly chosen folders.

left_folder = ‘Dots/stim_0’ + str(leftImage)
left_files = os.listdir(left_folder)
left_file = random.choice(left_files)
left_path = left_folder + ‘/’ + left_file

right_folder = ‘Dots/stim_0’ + str(rightImage)
right_files = os.listdir(right_folder)
right_file = random.choice(right_files)
right_path = right_folder + ‘/’ + right_file

I’m also attaching my conditions file in case it is useful.

dot_trials.xlsx (10.4 KB)

5

I’ve checked my records and found that I got a weird Key error of the next iteration number when using trials.finished = True in one particular experiment (sentence repetition) in a bespoke PsychoPy environment but it was fine in others using the same version. I fixed it by implementing trials.skipTrials(nConditions - trials.thisN).

Please could you send me your psyexp file so I can try to replicate the issue?

1 Like
6

oh thank you so much!!!

it might seem a bit messy but if you help me figure out what to do with the practice loop, hopefully i will take care of the rest! do i need to send other excel files or image folders as well?

indep.psyexp (196.0 KB)

7

heyyy again!

I implemented your solution and it worked! I used it as the following:

if practice.thisN == 4:
nConditions = len(practice.trialList)
practice.skipTrials(nConditions - practice.thisN)

thank you so much again!

8

I’m surprised that this bug hasn’t come up more often. Here’s a minimal demo that still shows it.

It seems to be related to saving the data from a loop and this still gives the error in our pre-release versions so it hasn’t been fixed yet. On the other hand, it doesn’t seem to always occur whenever .finished = True is used so there must be something different about this demo experiment from others I’ve tried.

loop-test.psyexp (22.4 KB)

dot_trials.csv (701 Bytes)

9

Well, I guess you’re right! I just changed the data-saving settings and “.finished = True” works without a problem when “Save Excel File” isn’t checked. I suppose this is an error about the new version of PsychoPy.

10

Yes – @TParsons has identified the cause and has a solution, but needs to check it doesn’t cause any unintended consequences.