docs: def fetch_user(query):
      """Get user by ``pk`` or ``username``. Raise error if it doesn't exist."""
      user_filter = {'pk': query} if query.isdigit() else {'username': query}
      user_model = get_user_model()
  
      try:
          return user_model.objects.get(**user_filter)
      except user_model.DoesNotExist:
          raise exceptions.ParseError("Unknown user: {}".format(query))