how to save a gtktextbuffer content in file

Question

i tried to save sengtktextbuffer content in a file. my code seens working but i have a problem in file.

some characters are unreadable in outfile outfile

my code:

def on_save_clicked(self, widget, data=None):
    start = self.textbuffer.get_start_iter()
    end = self.textbuffer.get_end_iter()

    this = self.textbuffer.get_text(start, end, False)
    format = self.textbuffer.register_serialize_tagset(this)
    data = self.textbuffer.serialize(self.textbuffer, format, start, end)

    outfile = open("/home/christophe/toto.txt", "w")
    outfile.write(data)
    outfile.close()

what is wrong in my code?

thanks for your help