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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 mutect2_caller(align_bams, items, ref_file, assoc_files, region=None, out_file=None):
 """Call variation with GATK's MuTect2. This requires the full non open-source version of GATK 3.5+. """

    if out_file is None:
        out_file = "%s-variants.vcf.gz" % utils.splitext_plus(align_bams[0])[0]
    if not utils.file_exists(out_file):
        paired = vcfutils.get_paired_bams(align_bams, items)
        broad_runner = broad.runner_from_config(items[0]["config"])
        gatk_type = broad_runner.gatk_type()
        _prep_inputs(align_bams, ref_file, items)
        with file_transaction(items[0], out_file) as tx_out_file:
            params = ["-T", "Mutect2" if gatk_type == "gatk4" else "MuTect2",
                      "--annotation", "ClippingRankSumTest",
                      "--annotation", "DepthPerSampleHC"]
            if gatk_type == "gatk4":
                params += ["--reference", ref_file]
            else:
                params += ["-R", ref_file]
            for a in annotation.get_gatk_annotations(items[0]["config"], include_baseqranksum=False):
                params += ["--annotation", a]
            # Avoid issues with BAM CIGAR reads that GATK doesn't like
            if gatk_type == "gatk4":
                params += ["--read-validation-stringency", "LENIENT"]
            params += _add_tumor_params(paired, items, gatk_type)
            params += _add_region_params(region, out_file, items, gatk_type)
            # Avoid adding dbSNP/Cosmic so they do not get fed to variant filtering algorithm
            # Not yet clear how this helps or hurts in a general case.
            #params += _add_assoc_params(assoc_files)
            resources = config_utils.get_resources("mutect2", items[0]["config"])
            if "options" in resources:
                params += [str(x) for x in resources.get("options", [])]
            assert LooseVersion(broad_runner.gatk_major_version()) >= LooseVersion("3.5"), \
                "Require full version of GATK 3.5+ for mutect2 calling"
            broad_runner.new_resources("mutect2")
            gatk_cmd = broad_runner.cl_gatk(params, os.path.dirname(tx_out_file))
            if gatk_type == "gatk4":
                tx_raw_prefilt_file = "%s-raw%s" % utils.splitext_plus(tx_out_file)
                tx_raw_file = "%s-raw-filt%s" % utils.splitext_plus(tx_out_file)
                filter_cmd = _mutect2_filter(broad_runner, tx_raw_prefilt_file, tx_raw_file, ref_file)
                cmd = "{gatk_cmd} -O {tx_raw_prefilt_file} && {filter_cmd}"
            else:
                tx_raw_file = "%s-raw%s" % utils.splitext_plus(tx_out_file)
                cmd = "{gatk_cmd} > {tx_raw_file}"
            do.run(cmd.format(**locals()), "MuTect2")
            out_file = _af_filter(paired.tumor_data, tx_raw_file, out_file)
    return vcfutils.bgzip_and_index(out_file, items[0]["config"])