python 遍历大文件，处理数据时，时时把变量保存到文件，不增大变量，节省内存

# 多用write()
def split_file(infile, n_parts, outdir):

    if not os.path.exists(infile):
        sys.stderr.write("Error: Can't find file: %s\n" % infile)
        sys.exit(1)

    fqname, ext = '', ''
    if infile.endswith(".fastq.gz"):
        fqname = os.path.basename(infile).split(".fastq.gz")[0]
        ext = "fastq.gz"
    elif infile.endswith(".fq.gz"):
        fqname = os.path.basename(infile).split(".fq.gz")[0]
        ext = "fq.gz"
    elif infile.endswith(".fastq"):
        fqname = os.path.basename(infile).split(".fastq")[0]
        ext = "fastq"
    elif infile.endswith(".fq"):
        fqname = os.path.basename(infile).split(".fq")[0]
        ext = "fq"
    else:
        sys.stderr.write("Error: The input files are not fastq format(*.fq.gz/*.fq/*.fastq.gz/*.fastq)\n")

    total_read_num, total_base_num = get_file_size(infile)

    elapsed_time = datetime.now() - START_TIME    
    print "Loaded %s: %d sequences, %d bp, %d seconds elapsed" % (infile, total_read_num, total_base_num, elapsed_time.seconds)
    print "=> dividing into %d parts:" % n_parts

    read_num_per_file = total_read_num/n_parts if total_read_num % n_parts == 0 else int(total_read_num/n_parts)+1
    num_len = len(str(n_parts))
    with gzip.open(infile) if infile.endswith(".gz") else open(infile) as I:
        for part in range(1, n_parts+1):
            part_file = "%s.%0*d.%s" % (fqname, num_len, part, ext)
            out_sub_file = '/'.join([outdir, part_file])
            
            print out_sub_file

            written = 0
            with gzip.open(out_sub_file, "wb") if out_sub_file.endswith(".gz") else open(out_sub_file, "w") as OUT:
                is_done = False
                while not is_done and written < read_num_per_file:
                    written += 1
                    is_done, _, read = get_fastq_read(I)
                    OUT.write("%s\n" % read)# 如这里