Arithmetic arranger(python)

I am working on arithmetic formatter project of scientific computing with
python. I had some error earlier, But I tried my best to complete this project.
No matter what I do, it will give me 6 errors. Test code should give 4 error and 3 formation( 2 without answer and 1 with answer). When I look at the console it shows it. But with bunch of error. In last it says, FAILED(failures = 6)

all the tests has AssertionError: None != "..."

the first element of the comparison is your output, meaning your function always return None instead of the requested values

But console is giving 3 formation and 4 error message, I didn’t ask for any ‘None’. What could be the problem and how to solve it…?