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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 variantcall_sample(data, region=None, align_bams=None, out_file=None):
 """Parallel entry point for doing genotyping of a region of a sample. """

    if out_file is None or not os.path.exists(out_file) or not os.path.lexists(out_file):
        utils.safe_makedir(os.path.dirname(out_file))
        ref_file = dd.get_ref_file(data)
        config = data["config"]
        caller_fns = get_variantcallers()
        caller_fn = caller_fns[config["algorithm"].get("variantcaller")]
        if len(align_bams) == 1:
            items = [data]
        else:
            items = multi.get_orig_items(data)
            assert len(items) == len(align_bams)
        assoc_files = tz.get_in(("genome_resources", "variation"), data, {})
        if not assoc_files: assoc_files = {}
        for bam_file in align_bams:
            bam.index(bam_file, data["config"], check_timestamp=False)
        out_file = caller_fn(align_bams, items, ref_file, assoc_files, region, out_file)
    if region:
        data["region"] = region
    data["vrn_file"] = out_file
    return [data]