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When I looked at my code block, I found that all tests had passed and each step had been run, but I got 0 points after submitting the job. The system told me "Cell #24. Can’t compile the student’s code. Error: TypeError(‘cannot unpack non-iterable NoneType object’)"and it doesn’t give where the error occurred.
Thanks for your review. Please help me solve the problem. Thank you
That means there is some section of code that you didn’t fill in that still contains “None” as a placeholder. But if you are sure you completed everything, then the issue is probably that you have not done a “Save” since the last time you changed the code. The grader does not do an “autosave” for you, so what it gets may not be what you are currently running. WYSINWTGG: What You See Is Not What The Grader Gets, unless you do “Save” first. I agree that this is a clear violation of the PLA (the Principle of Least Astonishment), but they didn’t ask my opinion. 
Thanks for your reply.
Yesterday, I found that it was indeed a problem of automatic saving, and it didn’t work after saving many times. However, today, when I visited my homework again, I found that the system can successfully save automatically, so I also got a score. Thank you again for your help