1.过程简述
1) 创建请求上下文
2) 创建应用上下文
3) 把上下文压入栈
4) 执行请求钩子 before_first_request 的相关操作
5) 执行请求钩子 before_request 的相关操作
6) 路由
7) 执行请求钩子 after_request 的相关操作
8) 执行请求钩子 teardown_request 的相关操作
9) 把上下文弹出栈
10) 返回响应结果
2.过程详述
2.1 wsgi 接口
总所周知，客户端每次发起请求，服务器都会调用框架实现的 wsgi 接口进行通讯。在 Flask 中，每个请求都会先调用 Flask.__call__ 方法，而此方法又调用了 Flask.wsgi_app ，它便是 Flask 中的 wsgi 接口了。接下来我们结合源码进行说明。更多精彩文章请关注公众号『Pythonnote』或者『全栈技术精选』
小闫使用的版本为
flask1.1.2，以下所有源码都源自于此版本，并以wsgi_app方法展开描述。
# flask/app.py class Flask(_PackageBoundObject): ... 省略其他方法 def wsgi_app(self, environ, start_response): """The actual WSGI application. ... 此处省略一些说明 :param environ: A WSGI environment. 环境字典，包含全部 HTTP 请求信息 :param start_response: A callable accepting a status code, a list of headers, and an optional exception context to start the response. 回调函数 """ # 创建请求上下文（过程中创建了应用上下文） ctx = self.request_context(environ) error = None try: try: # 上下文入栈 ctx.push() # 路由 response = self.full_dispatch_request() except Exception as e: error = e response = self.handle_exception(e) except: # noqa: B001 error = sys.exc_info()[1] raise # 返回响应结果 return response(environ, start_response) finally: if self.should_ignore_error(error): error = None # 上下文出栈 ctx.auto_pop(error) def __call__(self, environ, start_response): """The WSGI server calls the Flask application object as the WSGI application. This calls :meth:`wsgi_app` which can be wrapped to applying middleware.""" return self.wsgi_app(environ, start_response)
2.2 创建上下文
结合上面提供的 wsgi_app 源码，我们可以看到先执行了如下代码：
ctx = self.request_context(environ)
此处便是创建上下文操作。框架会先去创建请求上下文，并去判断是否有应用上下文，以及应用上下文与当前应用是否一致，然后决定是否去创建一个应用上下文。如下便是依次进行调用的方法：
# flask/app.py class Flask(_PackageBoundObject): def request_context(self, environ): return RequestContext(self, environ) # flask/ctx.py class RequestContext(object): def push(self): top = _request_ctx_stack.top if top is not None and top.preserved: top.pop(top._preserved_exc) # 从栈中弹出一个应用上下文 app_ctx = _app_ctx_stack.top # 判断应用上下文是否存在并与当前应用一致 if app_ctx is None or app_ctx.app != self.app: # 创建应用上下文 app_ctx = self.app.app_context() # 入栈 app_ctx.push() self._implicit_app_ctx_stack.append(app_ctx) else: self._implicit_app_ctx_stack.append(None) if hasattr(sys, "exc_clear"): sys.exc_clear() _request_ctx_stack.push(self) if self.session is None: session_interface = self.app.session_interface self.session = session_interface.open_session(self.app, self.request) if self.session is None: self.session = session_interface.make_null_session(self.app) if self.url_adapter is not None: self.match_request()
2.3 请求钩子
如果在代码中定义了四种请求钩子，那么它们会按照如下顺序执行。
a.接受一个参数：视图函数作出的响应 b.在此函数中可以对响应值,在返回之前做最后一步处理,再返回
1.before_first_request:在处理第一个请求前执行
2.before_request:在每次请求前执行,在该装饰函数中,一旦return,视图函数不再执行
3.after_request:如果没有抛出错误，在每次请求后执行
4.teardown_request：在每次请求后执行更多精彩文章请关注公众号『Pythonnote』或者『全栈技术精选』
a.接受一个参数:用来接收错误信息
2.4 路由
在 wsgi_app 方法中如下代码便会进行请求分发：
response = self.full_dispatch_request()
下面将所涉及到的方法源码依次列出：
# flask/app.py class Flask(_PackageBoundObject): def full_dispatch_request(self): """Dispatches the request and on top of that performs request pre and postprocessing as well as HTTP exception catching and error handling. """ # 首次处理请求前的操作，通过 @before_first_request 定义，也就是我们的请求钩子 self.try_trigger_before_first_request_functions() try: request_started.send(self) # 每次处理请求前进行的操作, 通过 @before_request 来定义 rv = self.preprocess_request() if rv is None: # 调用 dispatch_request 函数匹配 url，执行请求调度 rv = self.dispatch_request() except Exception as e: rv = self.handle_user_exception(e) # 调用 finalize_request 方法将视图函数的返回值转换成一个真正的响应对象 return self.finalize_request(rv) def dispatch_request(self): """Does the request dispatching. Matches the URL and returns the return value of the view or error handler. This does not have to be a response object. In order to convert the return value to a proper response object, call :func:`make_response`. """ req = _request_ctx_stack.top.request if req.routing_exception is not None: self.raise_routing_exception(req) rule = req.url_rule # if we provide automatic options for this URL and the # request came with the OPTIONS method, reply automatically if ( getattr(rule, "provide_automatic_options", False) and req.method == "OPTIONS" ): return self.make_default_options_response() # otherwise dispatch to the handler for that endpoint return self.view_functions[rule.endpoint](**req.view_args) def finalize_request(self, rv, from_error_handler=False): """Given the return value from a view function this finalizes the request by converting it into a response and invoking the postprocessing functions. This is invoked for both normal request dispatching as well as error handlers. Because this means that it might be called as a result of a failure a special safe mode is available which can be enabled with the `from_error_handler` flag. If enabled, failures in response processing will be logged and otherwise ignored. :internal: """ response = self.make_response(rv) try: # 每次正常处理请求后进行的操作, 通过 @after_request 来定义 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
2.5 路由表的创建
下面是一个最简单的视图，其作用便是在访问根目录时，返回 Hello World。
@app.route('/') def hello_world(): return 'Hello World!'
你有没有想过视图函数与 url 是如何绑定的呢？它是通过方法 add_url_rule 创建的路由规则，下面我们来看一下源码：
1) 先查看一下 @app.route 装饰器干了哪些工作：
# flask/app.py class Flask(_PackageBoundObject): def route(self, rule, **options): def decorator(f): # 获取 endpoint，可以看作为每个 view_func 的 ID endpoint = options.pop("endpoint", None) # 调用 add_url_rule 方法添加路由信息 self.add_url_rule(rule, endpoint, f, **options) return f return decorator
2) 然后就来看看 add_url_rule 为何方神圣。
# flask/app.py class Flask(_PackageBoundObject): # 定义view_functions self.view_functions = {} # 定义url_map self.url_map = Map() @setupmethod 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 # 创建 rule rule = self.url_rule_class(rule, methods=methods, **options) rule.provide_automatic_options = provide_automatic_options # 把 rule 添加到 url_map 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 ) # 把 view_func 添加到 view_functions 字典 self.view_functions[endpoint] = view_func
