given a GTF file, produce a new GTF file with only the longest transcript
    for each gene
    function lifted from:
    https://pythonhosted.org/gffutils/_modules/gffutils/helpers.html

def canonical_transcripts(gtf, out_file):
    """
    given a GTF file, produce a new GTF file with only the longest transcript
    for each gene
    function lifted from:
    https://pythonhosted.org/gffutils/_modules/gffutils/helpers.html
    """
    if file_exists(out_file):
        return out_file
    db = get_gtf_db(gtf)
    with file_transaction(out_file) as tx_out_file:
        with open(tx_out_file, "w") as out_handle:
            for gene in db.features_of_type('gene'):
                exon_list = []
                for ti, transcript in enumerate(db.children(gene, level=1)):
                    cds_len = 0
                    total_len = 0
                    exons = list(db.children(transcript, level=1))
                    for exon in exons:
                        exon_length = len(exon)
                        if exon.featuretype == 'CDS':
                            cds_len += exon_length
                        total_len += exon_length

                    exon_list.append((cds_len, total_len, transcript, exons))

                # If we have CDS, then use the longest coding transcript
                if max(i[0] for i in exon_list) > 0:
                    best = sorted(exon_list)[0]
                # Otherwise, just choose the longest
                else:
                    best = sorted(exon_list, key=lambda x: x[1])[0]
                for exon in db.children(best[2], level=1):
                    out_handle.write(str(exon) + "\n")
    return out_file