Pandas dataframe: set_index with inplace=True returns a NoneType, why?

Ok, now I understand, thanks for the comments!

So inplace=True should return None and make the change in the original object. It seemed that on listing the dataframe again, no changes were present.

But of course I should not have assigned the return value to the dataframe, i.e.

testDataframe = testDataframe.set_index(['Codering'], inplace=True)

should just be

testDataframe.set_index(['Codering'], inplace=True)

or

testDataframe = testDataframe.set_index(['Codering'], inplace=False)

otherwise the return value of the inplace index change (None) is the new content of the dataframe which is of course not the intend.

I am sure this is obvious to many and now it is to me as well but it wasn't without your help, thanks!!!