Saving variable value to a file does not work!!!!

[Husam]

Hi friends,
Im  a newbie and trying to save the value of variable 'counter' to a
file, but it does not work.
The code Im usig is:


for line in lines:                # I'm reading from file: test2.txt.
One of it's lines contain: ID 5.
    string.split(line)
    if line[0:2]=='ID':
        counter=int(line[3])
        output=open('test2.txt','a') # Hier Im opening the same file for
append.
        output.write(counter +1)            # This is the trouble making
line!
output.close()


The error message I get when this code is run:

Traceback (innermost last):
  File "./script.py", line 23, in ?
    output.write(counter+1)
TypeError: read-only buffer, int

Thanks in advance,