ValueError：混合迭代和读取方法会丢失数据

Question

我修改了我的代码，但又出现了另一个错误：

    fin=open("ah_d_01_CGATGT_L003_R1_001.fastq","r")
    fout=open("ah_d_01_CGATGT_L003_R1_001.filt.fastq","w")
    maxN=4
    contatutte=containcl=0
    seq=("")
    for line in fin:
         seq +=fin.read()
         ctrl=fin.read()
         qual=fin.read()
         countN=seq.count("N" ,0, len(seq))
        if maxN>countN:
           fout.write(line) # +"\n"
           fout.write(seq)
           fout.write(ctrl)
           fout.write(qual)
           containcl+=1
   fin.close
   fout.close
   print contatutte,containcl

错误：

  File "filtraN.py", line 7, in <module>
    seq +=fin.read()
ValueError: Mixing iteration and read methods would lose data