转载 Flask中客户端 - 服务器 - web应用程序 是如何处理request生成response的?

文章转载自https://blog.csdn.net/weixin_37923128/article/details/80992645 , 感谢原作者

当客户端向服务器发送一个请求时，服务器会将请求转发给web应用程序，应用程序处理完这个请求后将会返回一个response。在这篇文章我们分析一下flask怎样处理request，又是怎样生成response的，同时我们应该思考，在这个过程中，flask是怎样让url、endpoint、视图函数一一对应的。

一旦web应用接收到request，flask就会调用Flask类的call函数。在wsgi_app()函数中，我们看到，在调用了full_dispatch_request()函数后，就生成了response（Response类实例）。

 class Flask:
     def __call__(self, environ, start_response):
         return self.wsgi_app(environ, start_response)

     def wsgi_app(self, environ, start_response):
         ctx = self.request_context(environ)
         error = None
         try:
             try:
                 ctx.push()
                 response = self.full_dispatch_request()
             except Exception as e:
                 error = e
                 response = self.handle_exception(e)
            except:
                error = sys.exc_info()[1]
                raise
         return response(environ, start_response)
     finally:
         if self.should_ignore_error(error):
             error = None
         ctx.auto_pop(error)

在函数full_dispatch_request中，我们看到了dispatch_request函数，一路寻找下去，在这个函数里找到了view_functions这个字典。这个字典被多个函数在多个函数中出现。

 def full_dispatch_request(self):
     self.try_trigger_before_first_request_functions()
     try:
         request_started.send(self)
         rv = self.preprocess_request()
         if rv is None:
             rv = self.dispatch_request()
     except Exception as e:
         rv = self.handle_user_exception(e)
     return self.finalize_request(rv)

 def dispatch_request(self):
     req = _request_ctx_stack.top.request
     if req.routing_exception is not None:
     self.raise_routing_exception(req)
     rule = req.url_rule

     if getattr(rule, 'provide_automatic_options', False) \
         and req.method == 'OPTIONS':
         return self.make_default_options_response()
 return self.view_functions[rule.endpoint](**req.view_args)

 self.view_functions = {}

那这个add_url_rule函数又是在哪里被调用的呢？往下看。

 def add_url_rule(self, rule, endpoint=None, view_func=None,provide_automatic_options=None, **options):
     if endpoint is None:
         endpoint = _endpoint_from_view_func(view_func)
         options['endpoint'] = endpoint
         methods = options.pop('methods', None)

 # if the methods are not given and the view_func object knows its
 # methods we can use that instead. If neither exists, we go with
 # a tuple of only ``GET`` as default.
     if methods is None:
         methods = getattr(view_func, 'methods', None) or ('GET',)
     if isinstance(methods, string_types):
         raise TypeError('Allowed methods have to be iterables of strings, ''for example: @app.route(..., methods=["POST"])')
     methods = set(item.upper() for item in methods)

 # Methods that should always be added
     required_methods = set(getattr(view_func, 'required_methods', ()))

 # starting with Flask 0.8 the view_func object can disable and
 # force-enable the automatic options handling.
     if provide_automatic_options is None:
         provide_automatic_options = getattr(view_func,'provide_automatic_options', None)

     if provide_automatic_options is None:
         if 'OPTIONS' not in methods:
             provide_automatic_options = True
             required_methods.add('OPTIONS')
         else:
             provide_automatic_options = False

         # Add the required methods now.
         methods |= required_methods

         rule = self.url_rule_class(rule, methods=methods, **options)
         rule.provide_automatic_options = provide_automatic_options

         self.url_map.add(rule)
         if view_func is not None:
             old_func = self.view_functions.get(endpoint)
                 if old_func is not None and old_func != view_func:
                     raise AssertionError('View function mapping is overwriting an ''existing endpoint function: %s' % endpoint)
             self.view_functions[endpoint] = view_func

看到这里就应该明白了。在我们写的web应用程序中，往往需要对视图函数添加路由修饰，如：app.route(‘/’)。正是在路由函数中调用了add_url_rule函数，而这个函数负责绑定相应的视图函数、url、以及endpoint。由此，这三个参数建立联系。而后，在程序运行中，当web程序收到请求后，根据请求中的url找到endpoint，再根据endpoint找到视图函数，然后调用视图函数，在视图函数中，会处理那些需要处理的值（return self.view_functionsrule.endpoint）。接下来这个函数的返回值会交给finalize_request（）函数处理。

 def route(self, rule, **options):
     def decorator(f):
         endpoint = options.pop('endpoint', None)
         self.add_url_rule(rule, endpoint, f, **options)
         return f
     return decorator

正是在这个函数里，生成了response。（response是Response类的实例），最终，这个response被返回给客户端，然后显示为页面。

 def finalize_request(self, rv, from_error_handler=False):
     response = self.make_response(rv)
     try:
         response = self.process_response(response)
         request_finished.send(self, response=response)
     except Exception:
         if not from_error_handler:
         raise
             self.logger.exception('Request finalizing failed with an ''error while handling an error')
     return response