def convert_to_categorical(df):
        """Run a heuristic on the columns of the dataframe to determine whether it contains categorical values
           if the heuristic decides it's categorical then the type of the column is changed
        Args:
            df (dataframe): The dataframe to check for categorical data
        """
        might_be_categorical = df.select_dtypes(include=[object]).columns.tolist()
        for column in might_be_categorical:
            if df[column].nunique() < 20:

                # Convert the column
                print('Changing column {:s} to category...'.format(column))
                df[column] = pd.Categorical(df[column])
Run a heuristic on the columns of the dataframe to determine whether it contains categorical values
           if the heuristic decides it's categorical then the type of the column is changed
        Args:
            df (dataframe): The dataframe to check for categorical data