Problem of using gradethis for code checking

I found there is a limitation using gradethis package to assess the exercise code for python. It seems not agree to use a square bracket [ wrapped in a round bracket (. Even though I've set the solution in this way, it will still show message saying "A problem occurred with the grading code for this exercise". I guess it's because it can't evaluate python code properly.

An error will also show in my render console:

Another example is the code chunk to calculate mean value:

The error showing in my render console:

So I have to not using gradethis for now and possibly find another solution.

Start by reproducing the reported gradethis failures with the square-bracket expression and the mean calculation in the exercise render, then inspect the render console errors. Compare how gradethis evaluates these Python examples and identify whether the grading path can handle them; done means both examples render and grade without the reported error.