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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 _convert_fastq(srafn, outdir, single=False):
    "convert sra to fastq"
    cmd = "fastq-dump --split-files --gzip {srafn}"
    cmd = "%s %s" % (utils.local_path_export(), cmd)
    sraid = os.path.basename(utils.splitext_plus(srafn)[0])
    if not srafn:
        return None
    if not single:
        out_file = [os.path.join(outdir, "%s_1.fastq.gz" % sraid),
                    os.path.join(outdir, "%s_2.fastq.gz" % sraid)]
        if not utils.file_exists(out_file[0]):
            with utils.chdir(outdir):
                do.run(cmd.format(**locals()), "Covert to fastq %s" % sraid)
        if not utils.file_exists(out_file[0]):
            raise IOError("SRA %s didn't convert, something happened." % srafn)
        return [out for out in out_file if utils.file_exists(out)]
    else:
        raise ValueError("Not supported single-end sra samples for now.")