can anyone know about these
There must be some error in your notebook. Are you sure that you completed all the required portions? You usually get an error like this if you submit to the grader before finishing all the functions in the notebook. The error is pretty specific: search for the token “None” in your notebook. There must be someplace where you are referencing “None” and note that the other part of the clue is that the len() function is being applied to that variable. They use “None” as a “placeholder” for code you need to fill in. The other type of error that can cause this is referencing global variables that aren’t set in the grader context.
I left exercises 3 and 4 uncomplete. Thanks for your suggestions, i will look into it.
Yes, the general rule is that it does not do you any good to submit to the grader until you pass all the tests in the notebook. The reason is what you see here: if you get a syntax error because of the incomplete code, then you just get 0 for everything because the grader can’t even get as far as actually running any of your code.