I need to deal with super large txt input files, and I usually use .readlines() to first read the whole file, and turn it into a list.

I know it's really memory-cost and can be quite slow, but I also need to make use of LIST characteristics to manipulate the specific lines, like below:

```
#!/usr/bin/python

import os,sys
import glob
import commands
import gzip

path= '/home/xxx/scratch/'
fastqfiles1=glob.glob(path+'*_1.recal.fastq.gz')

for fastqfile1 in fastqfiles1:
    filename = os.path.basename(fastqfile1)
    job_id = filename.split('_')[0]
    fastqfile2 = os.path.join(path+job_id+'_2.recal.fastq.gz') 

    newfastq1 = os.path.join(path+job_id+'_1.fastq.gz') 
    newfastq2 = os.path.join(path+job_id+'_2.fastq.gz') 

    l1= gzip.open(fastqfile1,'r').readlines()
    l2= gzip.open(fastqfile2,'r').readlines()
    f1=[]
    f2=[]
    for i in range(0,len(l1)):
        if i % 4 == 3:
           b1=[ord(x) for x in l1[i]]
           ave1=sum(b1)/float(len(l1[i]))
           b2=[ord(x) for x in str(l2[i])]
           ave2=sum(b2)/float(len(l2[i]))
           if (ave1 >= 20 and ave2>= 20):
              f1.append(l1[i-3])
              f1.append(l1[i-2])
              f1.append(l1[i-1])
              f1.append(l1[i])
              f2.append(l2[i-3])
              f2.append(l2[i-2])
              f2.append(l2[i-1])
              f2.append(l2[i])
    output1=gzip.open(newfastq1,'w')
    output1.writelines(f1)
    output1.close()
    output2=gzip.open(newfastq2,'w')
    output2.writelines(f2)
    output2.close()

```

In general, I'm trying to read every 4th line of the whole text, but if the 4th line meets the desired condition, I'll append these 4 lines into the text.
So can I avoid readlines() to achieve this?
thx

EDIT:
Hi, actually I myself found a better way:

```
import commands
 l1=commands.getoutput('zcat ' + fastqfile1).splitlines(True)
 l2=commands.getoutput('zcat ' + fastqfile2).splitlines(True)

```

I think 'zcat' is super fast....
It took around 15min to readlines, while only 1 minute to just zcat...