code: def _validate_domains(domains):
          
          missing = set(settings.TRANSLATION_DOMAINS.keys()) - set(domains.keys())
          if missing:
              print('The following domains have been set in the configuration, '
                    'but their sources were not provided, use the `--source` '
                    'option to specify their sources: {domains}'.format(domains=', '.join(missing)))
              sys.exit(1)
docs: Check that all domains specified in the settings was provided in the options.