django之setup()

#django包的__init__.py包含setup函数
def setup():
    """
    Configure the settings (this happens as a side effect of accessing the
    first setting), configure logging and populate the app registry.
    """
    from django.apps import apps
    from django.conf import settings
    from django.utils.log import configure_logging

    configure_logging(settings.LOGGING_CONFIG, settings.LOGGING)
    apps.populate(settings.INSTALLED_APPS)#apps对象安装app
def populate(self, installed_apps=None):
        """
        Loads application configurations and models.

        This method imports each application module and then each model module.

        It is thread safe and idempotent, but not reentrant.
        """
        if self.ready:
            return

        # populate() might be called by two threads in parallel on servers
        # that create threads before initializing the WSGI callable.
        with self._lock:
            if self.ready:
                return

            # app_config should be pristine, otherwise the code below won't
            # guarantee that the order matches the order in INSTALLED_APPS.
            if self.app_configs:
                raise RuntimeError("populate() isn't reentrant")

            # Load app configs and app modules.
            for entry in installed_apps:
                if isinstance(entry, AppConfig):
                    app_config = entry
                else:
                    app_config = AppConfig.create(entry)#创建一个AppConfig对象
                if app_config.label in self.app_configs:
                    raise ImproperlyConfigured(
                        "Application labels aren't unique, "
                        "duplicates: %s" % app_config.label)

                self.app_configs[app_config.label] = app_config

            # Check for duplicate app names.
            counts = Counter( #判断有无重复的AppConfig对象对象
                app_config.name for app_config in self.app_configs.values())
            duplicates = [
                name for name, count in counts.most_common() if count > 1]
            if duplicates:
                raise ImproperlyConfigured(
                    "Application names aren't unique, "
                    "duplicates: %s" % ", ".join(duplicates))

            self.apps_ready = True

            # Load models.
            for app_config in self.app_configs.values():#为每个app导入models_module
                all_models = self.all_models[app_config.label]
                app_config.import_models(all_models)

            self.clear_cache()

            self.models_ready = True

            for app_config in self.get_app_configs():
                app_config.ready()

            self.ready = True

#以django.contrib.admin为例说明create过程
default_app_config = 'django.contrib.admin.apps.AdminConfig'

class SimpleAdminConfig(AppConfig):
    """Simple AppConfig which does not do automatic discovery."""

    name = 'django.contrib.admin' #作为app_name
    verbose_name = _("Administration")

    def ready(self):
        checks.register(check_admin_app, checks.Tags.admin)

class AdminConfig(SimpleAdminConfig):
    """The default AppConfig for admin which does autodiscovery."""

    def ready(self):
        super(AdminConfig, self).ready()
        self.module.autodiscover()

@classmethod #AppConfig类的创建方法
    def create(cls, entry):
        """
        Factory that creates an app config from an entry in INSTALLED_APPS.
        """
        try:
            # If import_module succeeds, entry is a path to an app module,
            # which may specify an app config class with default_app_config.
            # Otherwise, entry is a path to an app config class or an error.
            module = import_module(entry)#会导入包的__init__.py

        except ImportError:
            # Track that importing as an app module failed. If importing as an
            # app config class fails too, we'll trigger the ImportError again.
            module = None

            mod_path, _, cls_name = entry.rpartition('.')

            # Raise the original exception when entry cannot be a path to an
            # app config class.
            if not mod_path:
                raise

        else:
            try:
                # If this works, the app module specifies an app config class.
                entry = module.default_app_config#从模块对象中取得默认配置文件类
            except AttributeError:
                # Otherwise, it simply uses the default app config class.
                return cls(entry, module)
            else:
                mod_path, _, cls_name = entry.rpartition('.')#分解成路径和类名

        # If we're reaching this point, we must attempt to load the app config
        # class located at <mod_path>.<cls_name>
        mod = import_module(mod_path)#导入apps模块
        try:
            cls = getattr(mod, cls_name)#取得类对象，比如在admin中为AdminConfig
        except AttributeError:
            if module is None:
                # If importing as an app module failed, that error probably
                # contains the most informative traceback. Trigger it again.
                import_module(entry)
            else:
                raise

        # Check for obvious errors. (This check prevents duck typing, but
        # it could be removed if it became a problem in practice.)
        if not issubclass(cls, AppConfig):
            raise ImproperlyConfigured(
                "'%s' isn't a subclass of AppConfig." % entry)

        # Obtain app name here rather than in AppClass.__init__ to keep
        # all error checking for entries in INSTALLED_APPS in one place.
        try:
            app_name = cls.name#类对象中指定的name，比如name = 'django.contrib.admin'可以有'.'。
        except AttributeError:
            raise ImproperlyConfigured(
                "'%s' must supply a name attribute." % entry)

        # Ensure app_name points to a valid module.
        app_module = import_module(app_name)#再次导入模块

        # Entry is a path to an app config class.
        return cls(app_name, app_module)

现在的问题是什么时候导入models。