CS 124
Fall 2023

Problem Set 12 FAQ

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General questions

  1. I keep getting a NoneType error. Why does this happen?

    If your dates are suddenly becoming None and you do not know why, check over your code to make sure that you are not assigning the result of the advance_one method to a variable.

    x = d.advance_one()        # incorrect
self = self.advance_one()  # incorrect

    Remember, advance_one does not return a value. We call it to modify an object, and we do not want it to return anything. As a result, you should not assign the result of a call to advance_one to a variable. If you do, that variable will end up with a value of None (a special value that is returned when a function does not explicitly return a value), and that can then lead to a NoneType error.

    Instead, all of your calls to advance_one should occur all by themselves on their own lines:

    d.advance_one()            # correct
self.advance_one()         # correct

    This same rule also applies to calls to advance_n, since it should not return a value either.

  2. How do I read from a file?

    In the videos, we presented two different ways to read the contents of a file.

    In those examples, you will see that you can process a file line by line, or read the entire file as one big string.

Problem 2

See also question 1 from the general questions above.

  1. The logic of my days_between method looks right, but it’s giving the wrong answer. Do you have any suggestions?

    The days_between method should use other methods from your Date class. If your logic in days_between seems correct but you are still getting the wrong answer, you should check your methods from earlier in the problem to see if they are correct. Even if you pass the test cases we have given you for those other methods, there may still be a bug. For instance, try is_before with two different Date objects from the same year and month. Does the result seem right? What about for two Dates in different months?

  2. My days_between method hangs (i.e., it keeps running indefinitely without giving me a return value) when I call it for certain dates. What could the problem be?

    The problem could be in your advance_one method. Make sure that it actually advances the date in all possible cases. In particular, if you are using an if-elif statement, you may want to add an else case to ensure that the date is advanced no matter what. If advance_one fails to advance the date in certain cases, then your days_between method can end up in an infinite loop, which causes the method to hang.

    Another thing worth checking is whether your days_between method may be incorrectly advancing its copy of the later of the two dates, rather than its copy of the earlier of the two dates. This can happen if your is_before method is not returning the correct value when it compares the two dates in question.

  3. Is it good enough if my Date methods pass the test cases given in the assignment?

    Even if your Date methods pass the test cases given in the assignment, they may still be incorrect. We highly recommend thinking of your own test cases to ensure that your Date methods work properly. If you find a case in which a method gives invalid results, use temporary print statements to figure out where the unexpected behavior occurs.