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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 _prep_inputs(vrn_info, cnv_info, somatic_info, work_dir, config):
 """Prepare inputs for running PhyloWGS from variant and CNV calls. """

    exe = os.path.join(os.path.dirname(sys.executable), "create_phylowgs_inputs.py")
    assert os.path.exists(exe), "Could not find input prep script for PhyloWGS runs."
    ssm_file = os.path.join(work_dir, "ssm_data.txt")
    cnv_file = os.path.join(work_dir, "cnv_data.txt")
    if not utils.file_exists(ssm_file) or not utils.file_exists(cnv_file):
        with file_transaction(somatic_info.tumor_data, ssm_file, cnv_file) as (tx_ssm_file, tx_cnv_file):
            variant_type, input_vcf_file = _prep_vrn_file(vrn_info["vrn_file"], vrn_info["variantcaller"],
                                                          work_dir, somatic_info, cnv_info["ignore"], config)
            input_cnv_file = _prep_cnv_file(cnv_info["subclones"], work_dir, somatic_info)
            cmd = [sys.executable, exe,
                   "--sample-size", str(config["sample_size"]), "--tumor-sample", somatic_info.tumor_name,
                   "--battenberg", input_cnv_file, "--cellularity", _read_contam(cnv_info["contamination"]),
                   "--output-cnvs", tx_cnv_file, "--output-variants", tx_ssm_file,
                   "--variant-type", variant_type, input_vcf_file]
            do.run(cmd, "Prepare PhyloWGS inputs.")
    return ssm_file, cnv_file