Find a string and write it in a new text file with its next line using Python


I am writing a code in Python for searching a string in a huge text file which will occur every 10-15 lines and copying its next line in another text file. I am a beginner in Python so not sure what would be best to do this. I am trying by using the below script:

name = raw_input('Enter file:')
    with open(name) as f:
        with open("output.txt", "w") as f1:
            for line in f:
                if "IDENTIFIER" in line:

After this what I need in output file is the entire next line after this string is found.

something like line+1 which I suppose is not available in Python.

How can I jump to the next line and write that line in the output file after me text IDENTIFIER?

with open("file_in.txt") as f:
   with open("file_out.txt","w") as f2:
        for line in f:
            if "my_test" in line:
                f2.write(line.rstrip("\n")+next(f)) # the next line :P