Django源碼解析：setting.py

1. setting.py文件

咱們在django項目中，新建一個app的時候，都會有一個setting.py文件，裏面包含了整個項目全部的配置，包括apps，中間鍵，數據庫等等，django是如何將該setting文件利用起來的呢。

2. 從setting.py配置文件到Setting類

（1）啓動的時候，指定默認的配置文件

django在啓動的時候，會調用manage.py文件運行server的實例，在文件一開始的時候，manage.py會調用下面代碼

os.environ.setdefault("DJANGO_SETTINGS_MODULE", "djtest.settings")

設置默認的配置文件的環境變量DJANGO_SETTINGS_MODULE，該環境變量的名稱，定義在django/conf/_init.py文件裏面

ENVIRONMENT_VARIABLE = "DJANGO_SETTINGS_MODULE"

（2）根據配置文件裏面的配置項，提取到django/conf/_init.py裏面的Setting類裏面

過程主要包含兩步

1. 根據DJANGO_SETTINGS_MODULE指定的配置文件調用mod = importlib.import_module(self.SETTINGS_MODULE)，將該模塊import

2. 調用for setting in dir(mod)  遍歷該mod的屬性，將全部的大寫的配置屬性，提出取來，利用setattr(self, setting, setting_value)，保存爲本身的屬性。

 

class Settings(BaseSettings):
    def __init__(self, settings_module):
        # update this dict from global settings (but only for ALL_CAPS settings)
        for setting in dir(global_settings):
            if setting.isupper():
                setattr(self, setting, getattr(global_settings, setting))

        # store the settings module in case someone later cares
        self.SETTINGS_MODULE = settings_module

        try:
            # 根據 manage.py 中的os.environ.setdefault("DJANGO_SETTINGS_MODULE", "djtest.settings")，設置的配置文件，import該配置文件
            mod = importlib.import_module(self.SETTINGS_MODULE)
        except ImportError as e:
            raise ImportError(
                "Could not import settings '%s' (Is it on sys.path? Is there an import error in the settings file?): %s"
                % (self.SETTINGS_MODULE, e)
            )


        tuple_settings = ("INSTALLED_APPS", "TEMPLATE_DIRS")
        # setting文件裏面顯示申明的屬性集合，是個set類型的，是個集合
        self._explicit_settings = set()
        # dir（object）返回object的屬性方法列表，所以，這裏dir會返回setting文件裏面的全部等號左邊的屬性
        for setting in dir(mod):
            if setting.isupper():
                setting_value = getattr(mod, setting)

                if (setting in tuple_settings and
                        isinstance(setting_value, six.string_types)):
                    raise ImproperlyConfigured("The %s setting must be a tuple. "
                            "Please fix your settings." % setting)
                # 將setting文件裏面的全部屬性提出來，放到這個Setting類裏面來
                setattr(self, setting, setting_value)
                self._explicit_settings.add(setting)

        if not self.SECRET_KEY:
            raise ImproperlyConfigured("The SECRET_KEY setting must not be empty.")

        if hasattr(time, 'tzset') and self.TIME_ZONE:
            # When we can, attempt to validate the timezone. If we can't find
            # this file, no check happens and it's harmless.
            zoneinfo_root = '/usr/share/zoneinfo'
            if (os.path.exists(zoneinfo_root) and not
                    os.path.exists(os.path.join(zoneinfo_root, *(self.TIME_ZONE.split('/'))))):
                raise ValueError("Incorrect timezone setting: %s" % self.TIME_ZONE)
            # Move the time zone info into os.environ. See ticket #2315 for why
            # we don't do this unconditionally (breaks Windows).
            os.environ['TZ'] = self.TIME_ZONE
            time.tzset()
            
    def is_overridden(self, setting):
        return setting in self._explicit_settings

（3）django的其餘模塊在使用配置文件裏面的配置項的時候，就是從Setting類中獲取，而不從配置文件直接獲取。