How to call function that is in the other class python for pytest

I am trying to call the methods in CSVDatasource in my testing class by typing this code from ETL.CSVDatasource import CSVDatasource and to call the necessary methods but I have been receiving errors like ** TypeError: unbound method preprocess_col() must be called with CSVDatasource instance as first argument (got DataFrame instance instead)**

http://ift.tt/1Jm6S1M -> Image of my coding path

Anyone can guide me on calling out the method in the other class so that I can call the method and do testing in my testing classs? Thanks.