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)))
