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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 _normalize_files(item, fc_dir=None):
 """Ensure the files argument is a list of absolute file names. Handles BAM, single and paired end fastq, as well as split inputs. """

    files = item.get("files")
    if files:
        if isinstance(files, six.string_types):
            files = [files]
        fastq_dir = flowcell.get_fastq_dir(fc_dir) if fc_dir else os.getcwd()
        files = [_file_to_abs(x, [os.getcwd(), fc_dir, fastq_dir]) for x in files]
        files = [x for x in files if x]
        _sanity_check_files(item, files)
        item["files"] = files
    return item