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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 _create_pileup(bam_file, data, out_base, background):
 """Create pileup calls in the regions of interest for hg19 -> GRCh37 chromosome mapping. """

    out_file = "%s-mpileup.txt" % out_base
    if not utils.file_exists(out_file):
        with file_transaction(data, out_file) as tx_out_file:
            background_bed = os.path.normpath(os.path.join(
                os.path.dirname(os.path.realpath(utils.which("verifybamid2"))),
                "resource", "%s.%s.%s.vcf.gz.dat.bed" % (background["dataset"],
                                                         background["nvars"], background["build"])))
            local_bed = os.path.join(os.path.dirname(out_base),
                                     "%s.%s-hg19.bed" % (background["dataset"], background["nvars"]))
            if not utils.file_exists(local_bed):
                with file_transaction(data, local_bed) as tx_local_bed:
                    with open(background_bed) as in_handle:
                        with open(tx_local_bed, "w") as out_handle:
                            for line in in_handle:
                                out_handle.write("chr%s" % line)
            mpileup_cl = samtools.prep_mpileup([bam_file], dd.get_ref_file(data), data["config"], want_bcf=False,
                                                target_regions=local_bed)
            cl = ("{mpileup_cl} | sed 's/^chr//' > {tx_out_file}")
            do.run(cl.format(**locals()), "Create pileup from BAM input")
    return out_file