def _get_gtf_column(column_name, gtf_path, df):
    """
    Helper function which returns a dictionary column or raises an ValueError
    abou the absence of that column in a GTF file.
    """
    if column_name in df.columns:
        return list(df[column_name])

    else:
        raise ValueError(
            "Missing '%s' in columns of %s, available: %s" % (
                column_name,
                gtf_path,
                list(df.columns)))