registry.py 文件源码

def get_swappable_settings_name(self, to_string):
        """
        For a given model string (e.g. "auth.User"), return the name of the
        corresponding settings name if it refers to a swappable model. If the
        referred model is not swappable, return None.

        This method is decorated with lru_cache because it's performance
        critical when it comes to migrations. Since the swappable settings don't
        change after Django has loaded the settings, there is no reason to get
        the respective settings attribute over and over again.
        """
        for model in self.get_models(include_swapped=True):
            swapped = model._meta.swapped
            # Is this model swapped out for the model given by to_string?
            if swapped and swapped == to_string:
                return model._meta.swappable
            # Is this model swappable and the one given by to_string?
            if model._meta.swappable and model._meta.label == to_string:
                return model._meta.swappable
        return None