Read every fourth line in Python

Question

I have a file where I am trying to read (and write to another file) every fourth line. The solution I'm using doesn't give the desired outcome.

sample.txt line1 line2 line3 line4 line5 line6 line7 line8 line9 line10

script.py

from itertools import islice
with open('sample.txt') as fin, open('output.txt','w') as fout:
         fout.writelines(islice(fin,None,None,4))

Now the output file gives me line1, line5, line9 instead of line4, line8.

How would I go about altering this to get the desired outcome?

Answer 1

You're almost there. islice comes with a start parameter. In your case, it's the second one, that you've set to None. Try this:

fout.writelines(islice(fin,3,None,4)) # line with index 3 is the fourth line