How to read 2 lines separately in python

Question

This is my first question here!

I have a file contains hundreds of lines such as:

<car> <part_of> <machine>     
<motor> <part_of> <car>       

Each line represents subject, relation, object

I want to read each 2 lines separately, process them, then output 2 or 3 lines based on the input file. I did something like this:

    opener = open('input.txt')        # to read even lines
    opener2 = open('input.txt')       # to read odd lines
    num = 2
    for eachline in opener:
        if num % 2 == 0:
            line1 = opener.readline().split()
            sub_line1, rel_line1, obj_line1 = line1[0],line1[1],line1[2]
            sub_line1 = line1[0].lstrip("<").rstrip(">")
            rel_line1 = line1[1].lstrip("<").rstrip(">")
            obj_line1 = line1[2].lstrip("\"").rstrip("\"")
        else:
            line2 = opener2.readline().split()
            sub_line2, rel_line2, obj_line2 = line1[0],line2[1],line2[2]
            sub_line2 = line2[0].lstrip("<").rstrip(">")
            rel_line2 = line2[1].lstrip("<").rstrip(">")
            obj_line2 = line2[2].lstrip("\"").rstrip("\"")
        num += 1

And I did this for the output:

output1 = " ".join([sub_line1,rel_line1,obj_line1])
writer.write(output1+"\n")
output2 = " ".join([sub_line2,rel_line2,obj_line2])
writer.write(output2+'\n')
output3 = " ".join([sub_line1,relation,sub_line2])
writer.write(output3+'\n')

Note: output3 does not exist in the input. I am adding this by combining the previous 2 lines. But every time the odd lines rewritten by the even one. How can I separate them ?

sample output:

<car> <part_of> <machine>
<motor> <part_of> <car>
<car> <part_of> <motor>

each 3rd line is composed based on the previous 2 lines.

PART 2:

If there is a line in the input file, starts with "_" how can I just output it as it is without including it as one of the 2 lines I am processing? where can I put this condition?

Thanks in advance!!

Answer 1

You cannot get a file descriptor (the stuff returned by open) to read only odd or even lines. It has to read the whole content of the file¹.

However and therefore, you do not need to have two file descriptors: you can do the job with only one. You can iterate over enumerate(file) instead of file. Instead of giving you the lines, it will give you (index, line) couples. You can unpack this by doing for id, line in enumerate(file), and then check the remainder of id by 2 to determine if it's odd or even.

file = open(path, 'r')
for id, line in enumerate(file):
    if id % 2 == 0:
        # The line is even
    else:
        # The line is odd

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¹To be fair, you could get a file descriptor to read only odd or even lines, in that you could just skip every other line... But then, why bother to create two descriptors when a single one is already doing the job?