Django中间件初始化过程（草稿）

def load_middleware(self):     """     Populate middleware lists from settings.MIDDLEWARE.      Must be called after the environment is fixed (see __call__ in subclasses).     """     self._view_middleware = []     self._template_response_middleware = []     self._exception_middleware = []      handler = convert_exception_to_response(self._get_response) #为返回视图的函数，包一层出错函数处理异常。如果出错，返回出错的response，如果没出错，返回_get_response返回的东西     for middleware_path in reversed(settings.MIDDLEWARE):         middleware = import_string(middleware_path)         try:             mw_instance = middleware(handler)         except MiddlewareNotUsed as exc:             if settings.DEBUG:                 if str(exc):                     logger.debug(‘MiddlewareNotUsed(%r): %s‘, middleware_path, exc)                 else:                     logger.debug(‘MiddlewareNotUsed: %r‘, middleware_path)             continue          if mw_instance is None:             raise ImproperlyConfigured(                 ‘Middleware factory %s returned None.‘ % middleware_path             )          if hasattr(mw_instance, ‘process_view‘):             self._view_middleware.insert(0, mw_instance.process_view)         if hasattr(mw_instance, ‘process_template_response‘):             self._template_response_middleware.append(mw_instance.process_template_response)         if hasattr(mw_instance, ‘process_exception‘):             self._exception_middleware.append(mw_instance.process_exception)          handler = convert_exception_to_response(mw_instance)      # We only assign to this when initialization is complete as it is used     # as a flag for initialization being complete.     self._middleware_chain = handler   

def _get_response(self, request):    """    Resolve and call the view, then apply view, exception, and    template_response middleware. This method is everything that happens    inside the request/response middleware.    """    response = None    if hasattr(request, ‘urlconf‘):        urlconf = request.urlconf        set_urlconf(urlconf)        resolver = get_resolver(urlconf)    else:        resolver = get_resolver()      resolver_match = resolver.resolve(request.path_info)  #从请求url获取视图    callback, callback_args, callback_kwargs = resolver_match  #获取视图函数，以及传参    request.resolver_match = resolver_match    # Apply view middleware    for middleware_method in self._view_middleware:        response = middleware_method(request, callback, callback_args, callback_kwargs) #在调用视图前调用view中间件，注意，如果有返回response，将不调用视图函数        if response:            break    if response is None:        wrapped_callback = self.make_view_atomic(callback)  #为每个视图封装一个事务        try:            response = wrapped_callback(request, *callback_args, **callback_kwargs)  #调用视图函数        except Exception as e:            response = self.process_exception_by_middleware(e, request)    # Complain if the view returned None (a common error).   #调用视图之后返回response为空的异常处理    if response is None:        if isinstance(callback, types.FunctionType):    # FBV            view_name = callback.__name__        else:      # CBV            view_name = callback.__class__.__name__ + ‘.__call__‘        raise ValueError(            "The view %s.%s didn‘t return an HttpResponse object. It "            "returned None instead." % (callback.__module__, view_name)        )    # If the response supports deferred rendering, apply template    # response middleware and then render the response    elif hasattr(response, ‘render‘) and callable(response.render):        for middleware_method in self._template_response_middleware:            response = middleware_method(request, response)            # Complain if the template response middleware returned None (a common error).            if response is None:                raise ValueError(                    "%s.process_template_response didn‘t return an "                    "HttpResponse object. It returned None instead."                    % (middleware_method.__self__.__class__.__name__)                )        try:            response = response.render()        except Exception as e:            response = self.process_exception_by_middleware(e, request)    return response

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