How do I delete 4 characters from the end of certain lines in a string?

I've got an .obj file, which is a text file used for 3D graphics, and here's a sample:

f 17439/17439 17440/17440 17441/17441
g lCollar_306
f 17442/17442 17443/17443 17444/17444
f 17445/17445 17446/17446 17447/17447
f 17448/17448 17449/17449 17450/17450
f 17451/17451 17452/17452 17453/17453
f 17454/17454 17455/17455 17456/17456
f 17457/17457 17458/17458 17459/17459
g lShldr_308
f 17460/17460 17461/17461 17462/17462
f 17463/17463 17464/17464 17465/17465
g lCollar_306

I want to go through the file, find each line that starts with the letter 'g', and delete the last four characters from each of those lines (removing the underscore and the 3-digit number).

I've tried the following:

for line in data.split('\n'):
    if (line.startswith('g')):
        line = line.rstrip(line[-4])

That doesn't seem to change anything in the variable data. I need to get the output as a string that I can save to a file, but I can't seem to get it to work. How do I fix this?

Solution

You're splitting your buffer into lines and iterating over them, assigning each one to the variable line. Then you generate a new string (rstrip) and assign it back to the variable line. But that doesn't change the original buffer. You're just generating a new string off to the side and then discarding it.

What you probably want to do is write the lines out to a new file, something like this:

f = open("output.obj", "w")
for line in data.split('\n'):
    if line.startswith('g'):
        line = line.rstrip(line[-4])
    print(line, file=f, end='\n')
f.close()

Notice that here we're doing something with the new line value (after calling rstrip).