code: def cmd(self, *args, **kwargs):
          '''adb command, add -s serial by default. return the subprocess.Popen object.'''
          serial = self.device_serial()
          if serial:
              if " " in serial:  # TODO how to include special chars on command line
                  serial = "'%s'" % serial
              return self.raw_cmd(*["-s", serial] + list(args))
          else:
              return self.raw_cmd(*argsf cmd(self, *args, **kwargs):
          '''adb command, add -s serial by default. return the subprocess.Popen object.'''
          serial = self.device_serial()
          if serial:
              if " " in serial:  # TODO how to include special chars on command line
                  serial = "'%s'" % serial
              return self.raw_cmd(*["-s", serial] + list(args))
          else:
              return self.raw_cmd(*args)
docs: adb command, add -s serial by default. return the subprocess.Popen object.