code: def populate_shell_combo(self):
          
          cb = self.get_widget('default_shell')
          # append user shell as first option
          cb.append_text(USER_SHELL_VALUE)
          if os.path.exists(SHELLS_FILE):
              lines = open(SHELLS_FILE).readlines()
              for i in lines:
                  possible = i.strip()
                  if possible and not possible.startswith('#') and os.path.exists(possible):
                      cb.append_text(possible)
  
          for i in get_binaries_from_path(PYTHONS):
              cb.append_text(i)
docs: Read the /etc/shells and looks for installed shells to
          fill the default_shell combobox.