I want to do 10-fold cross-validation for huge files ( running into hundreds of thousands of lines each). I want to do a "wc -l " each time i start reading a file, then generate random numbers a fixed number of times, each time writing that line number into a separate file . I am using this: 

```
import os 
for i in files:
    os.system("wc -l <insert filename>").

```

How do I insert the file name there. Its a variable. I went through the documentation but they mostly list out `ls` commands, something that doesn't have this problem.