docs: def get_qr(self, filename=None):
          """Get pairing QR code from client"""
          if "Click to reload QR code" in self.driver.page_source:
              self.reload_qr()
          qr = self.driver.find_element_by_css_selector(self._SELECTORS['qrCode'])
          if filename is None:
              fd, fn_png = tempfile.mkstemp(prefix=self.username, suffix='.png')
          else:
              fd = os.open(filename, os.O_RDWR | os.O_CREAT)
              fn_png = os.path.abspath(filename)
          self.logger.debug("QRcode image saved at %s" % fn_png)
          qr.screenshot(fn_png)
          os.close(fd)
          return fn_png