Django 配置文件的实现（从 0 到 1 实现）

Django配置文件的实现基本原理

当Django在使用配置文件的时候，会创建一个LazySetting的对象。在这个类的_setup方法里面，创建一个Settings的实例。在我们创建Setting实例的时候，需要传入一个配置文件的环境，然后会将配置文件的内容，当作这个类的属性

# 配置文件的环境变量
ENVIRONMENT_VARIABLE = "DJANGO_SETTINGS_MODULE"

class LazySettings(LazyObject):
    """
    A lazy proxy for either global Django settings or a custom settings object.
    The user can manually configure settings prior to using them. Otherwise,
    Django uses the settings module pointed to by DJANGO_SETTINGS_MODULE.
    """

    def _setup(self, name=None):
        """
        Load the settings module pointed to by the environment variable. This
        is used the first time settings are needed, if the user hasn't
        configured settings manually.
        """
        settings_module = os.environ.get(ENVIRONMENT_VARIABLE)
        if not settings_module:
            desc = ("setting %s" % name) if name else "settings"
            raise ImproperlyConfigured(
                "Requested %s, but settings are not configured. "
                "You must either define the environment variable %s "
                "or call settings.configure() before accessing settings."
                % (desc, ENVIRONMENT_VARIABLE))
        # 在这里返回了一个settings对象，配置文件里面的内容，都会以属性的方法，挂载在这个类的实例上面
        self._wrapped = Settings(settings_module)

    def __repr__(self):
        # Hardcode the class name as otherwise it yields 'Settings'.
        if self._wrapped is empty:
            return '<LazySettings [Unevaluated]>'
        return '<LazySettings "%(settings_module)s">' % {
            'settings_module': self._wrapped.SETTINGS_MODULE,
        }

    def __getattr__(self, name):
        """Return the value of a setting and cache it in self.__dict__."""
        if self._wrapped is empty:
            self._setup(name)
        val = getattr(self._wrapped, name)
        self.__dict__[name] = val
        return val

    def __setattr__(self, name, value):
        """
        Set the value of setting. Clear all cached values if _wrapped changes
        (@override_settings does this) or clear single values when set.
        """
        if name == '_wrapped':
            self.__dict__.clear()
        else:
            self.__dict__.pop(name, None)
        super().__setattr__(name, value)

    def __delattr__(self, name):
        """Delete a setting and clear it from cache if needed."""
        super().__delattr__(name)
        self.__dict__.pop(name, None)

    def configure(self, default_settings=global_settings, **options):
        """
        Called to manually configure the settings. The 'default_settings'
        parameter sets where to retrieve any unspecified values from (its
        argument must support attribute access (__getattr__)).
        """
        if self._wrapped is not empty:
            raise RuntimeError('Settings already configured.')
        holder = UserSettingsHolder(default_settings)
        for name, value in options.items():
            if not name.isupper():
                raise TypeError('Setting %r must be uppercase.' % name)
            setattr(holder, name, value)
        self._wrapped = holder

    @staticmethod
    def _add_script_prefix(value):
        """
        Add SCRIPT_NAME prefix to relative paths.

        Useful when the app is being served at a subpath and manually prefixing
        subpath to STATIC_URL and MEDIA_URL in settings is inconvenient.
        """
        # Don't apply prefix to absolute paths and URLs.
        if value.startswith(('http://', 'https://', '/')):
            return value
        from django.urls import get_script_prefix
        return '%s%s' % (get_script_prefix(), value)

    @property
    def configured(self):
        """Return True if the settings have already been configured."""
        return self._wrapped is not empty

    @property
    def PASSWORD_RESET_TIMEOUT_DAYS(self):
        stack = traceback.extract_stack()
        # Show a warning if the setting is used outside of Django.
        # Stack index: -1 this line, -2 the caller.
        filename, _, _, _ = stack[-2]
        if not filename.startswith(os.path.dirname(django.__file__)):
            warnings.warn(
                PASSWORD_RESET_TIMEOUT_DAYS_DEPRECATED_MSG,
                RemovedInDjango40Warning,
                stacklevel=2,
            )
        return self.__getattr__('PASSWORD_RESET_TIMEOUT_DAYS')

    @property
    def STATIC_URL(self):
        return self._add_script_prefix(self.__getattr__('STATIC_URL'))

    @property
    def MEDIA_URL(self):
        return self._add_script_prefix(self.__getattr__('MEDIA_URL'))

# 配置文件的类
class Settings:
    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

        mod = importlib.import_module(self.SETTINGS_MODULE)

        tuple_settings = (
            "INSTALLED_APPS",
            "TEMPLATE_DIRS",
            "LOCALE_PATHS",
        )
        self._explicit_settings = set()
        for setting in dir(mod):
            if setting.isupper():
                setting_value = getattr(mod, setting)

                if (setting in tuple_settings and
                        not isinstance(setting_value, (list, tuple))):
                    raise ImproperlyConfigured("The %s setting must be a list or a tuple. " % setting)
                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 self.is_overridden('PASSWORD_RESET_TIMEOUT_DAYS'):
            if self.is_overridden('PASSWORD_RESET_TIMEOUT'):
                raise ImproperlyConfigured(
                    'PASSWORD_RESET_TIMEOUT_DAYS/PASSWORD_RESET_TIMEOUT are '
                    'mutually exclusive.'
                )
            setattr(self, 'PASSWORD_RESET_TIMEOUT', self.PASSWORD_RESET_TIMEOUT_DAYS * 60 * 60 * 24)
            warnings.warn(PASSWORD_RESET_TIMEOUT_DAYS_DEPRECATED_MSG, RemovedInDjango40Warning)

        if self.is_overridden('DEFAULT_HASHING_ALGORITHM'):
            warnings.warn(DEFAULT_HASHING_ALGORITHM_DEPRECATED_MSG, RemovedInDjango40Warning)

        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 = Path('/usr/share/zoneinfo')
            zone_info_file = zoneinfo_root.joinpath(*self.TIME_ZONE.split('/'))
            if zoneinfo_root.exists() and not zone_info_file.exists():
                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

    def __repr__(self):
        return '<%(cls)s "%(settings_module)s">' % {
            'cls': self.__class__.__name__,
            'settings_module': self.SETTINGS_MODULE,
        }

自己用代码实现Django加载配置文件

# manager.py文件的内容
import os

from moni_django.set_config import Setting

"""
   1. 模拟Django的manage.py文件
   2. 创建一个Setting的对象
   """
SETTING_MODEL = os.environ.setdefault('SETTING_MODEL', 'moni_django.settings')
setting = Setting(SETTING_MODEL)

# 配置文件类
import sys
import os
import importlib
from moni_django.global_settings import global_setting

# 从零开始，搭建Django的加载配置文件
class Setting(object):
    def __init__(self, settings):
        """
        1. settings 是我们在外面的设置
        2. 从环境变量里面进行获取setting文件的内容
        3. 实例化对象，然后给对象进行设置值
        4. 进行从环境变量里面进行取值
        """

        self.global_setting = global_setting
        for i in dir(global_setting):
            # 只有当写的才会会变成这个类的属性
            if i.isupper():
                setattr(self, i, getattr(global_setting, i))

        # 使用一个值来接收配置文件的环境变量
        self.SETTINGS_MODULE = settings
        # 项目更目录的配置文件
        mod = importlib.import_module(self.SETTINGS_MODULE)
        for i in dir(mod):
            if i.isupper():
                # 只有当配置文件的内容是大写的时候，才可以变成这个实例的属性
                # 接收一下值
                # 进行赋值操作
                setting_value = getattr(mod, i)
                setattr(self, i, setting_value)

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Django配置文件的实现（从0到1实现）

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