Problem with gzip module and windows

[Meles MELES]

Hi,
   I'm actually having a little problem with the gzip module under 
Windows.

I'm currently getting some text file containing many lines, all having 
the same format.

Before applying any treatment to them, I would like to stock them in a 
single compressed text file.

To do this, I've written a little piece of software like this one:

import gzip
Z1=gzip.GzipFile("test.txt.gz","ab",9)
for i in listoffile:
    f=open(i,"r")
    for line in f.readlines():
        Z1.write(line)
    f.close()
Z1.close()

Under linux, this little bit of software works very well. 
Under windows, it works very well the first time I launch the program 
(the test.txt.gz file is created and is filled with the lines of the 
other text files).
But when I launch the script a second time, it doesn't work. There is no 
error message while running, but no new lines are added to the 
compressed archive. I just get 2 or 3 lines of weird symbol at the end 
ofthe previously imported lines.

Any ideas of what's wrong?

Best regards.

Blaise.
-- 
La beauté sans la grâce attire, mais elle ne sait pas retenir ; 
c'est un appât sans hameçon. Maxime Planude