def read_data_file(self, fn):
        """returns the content of the file 'fn' in the data sibling dir."""
        fp = os.path.join(self.get_data_path(), fn)
        with open(fp, "r") as fin:
            content = fin.read()
        return content
returns the content of the file 'fn' in the data sibling dir.
tests/cat/test_cat.py