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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_bams():
 """ Download BAM files if needed, extract only chr17 reads, and regenerate .bai """

    for size, md5, url in bams:
        bam = os.path.join(
            args.data_dir,
            os.path.basename(url).replace('.bam', '_%s.bam' % CHROM))
        if not _up_to_date(md5, bam):
            logger.info(
                'Downloading reads on chromosome %s from %s to %s'
                % (CHROM, url, bam))
            cmds = ['samtools', 'view', '-b', url, COORD, '>', bam]
            logged_command(cmds)
        bai = bam + '.bai'
        if not os.path.exists(bai):
            logger.info('indexing %s' % bam)
            logger.info(' '.join(cmds))
            cmds = [
                'samtools',
                'index',
                bam]
            logged_command(cmds)
        if os.path.exists(os.path.basename(url) + '.bai'):
            os.unlink(os.path.basename(url) + '.bai')

    for size, md5, fn in bais:
        if not _up_to_date(md5, fn):
            cmds = [
                'samtools', 'index', bai.replace('.bai', '')]
            logged_command(cmds)