code: def _get_valid_with_defaults_modes(capabilities):
      
      capability = capabilities[":with-defaults"]
  
      try:
          valid_modes = [capability.parameters["basic-mode"]]
      except KeyError:
          raise WithDefaultsError(
              "Invalid 'with-defaults' capability URI advertised by the server; "
              "missing 'basic-mode' parameter"
          )
  
      try:
          also_supported = capability.parameters["also-supported"]
      except KeyError:
          return valid_modes
  
      valid_modes.extend(also_supported.split(","))
  
      return valid_modes
docs: Reference: https://tools.ietf.org/html/rfc6243#section-4.3