Flask中一次请求到响应的流程

1、首先根据WSGI发送的environ变量获取请求上下文，主要是根据函数ctx = self.request_context(environ)，然后将该请求上下文推入全局变量_request_ctx_stack中（ctx.push()).

12	def request_context(self, environ):    return RequestContext(self, environ)

2、得到请求后，要触发第一次请求函数，如果是该应用是第一次实例化，并存在第一次请求之前函数（存在before_first_req_func字典中中)，会调用存在该字典中的函数。当然一个实例也只执行一次，即在初始化的时候执行。

3、会发送请求开始信号，request_started，告知subscriber请求开始了。

4、如果存在before_request装饰的函数（函数位置在before_request_func字典中）,那么在调用正常请求前会调用该函数。

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@setupmethoddef before_request(self, f):    \”\”\”Registers a function to run before each request.The function will be called without any arguments.If the function returns a non-None value, it\’s handled asif it was the return value from the view and furtherrequest handling is stopped.\”\”\”self.before_request_funcs.setdefault(None, []).append(f)return f

从函数可以看到，这是一个装饰器,被该装饰器修饰的函数会添加到字典中。

5、调用正常的请求，返回一个该请求函数的值。 调用请求的源代码：

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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.requestif 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 automaticallyif getattr(rule, \’provide_automatic_options\’, False)    and req.method == \’OPTIONS\’:    return self.make_default_options_response()# otherwise dispatch to the handler for that endpointreturn self.view_functions[rule.endpoint](**req.view_args)

6、将请求函数返回值构造成响应类。

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def make_response(self, rv):    \”\”\”Converts the return value from a view function to a real    response object that is an instance of :attr:response_class.The following types are allowed for `rv`:  ======================= ===========================================:attr:`response_class`  the object is returned unchanged:class:`str`            a response object is created with the                        string as body:class:`unicode`        a response object is created with thequest_ctx_stack中（ctx.push()). 12 def request_context(self, environ):    return RequestContext(self, environ) 2、得到请求后，要触发第一次请求函数，如果是该应用是第一次实例化，并存在第一次请求之前函数（存在before_first_req_func字典中中)，会调用存在该字典中的函数。当然一个实例也只执行一次，即在初始化的时候执行。 3、会发送请求开始信号，request_started，告知subscriber请求开始了。 4、如果存在before_request装饰的函数（函数位置在before_request_func字典中）,那么在调用正常请求前会调用该函数。 12345678910 @setupmethoddef before_request(self, f):    \”\”\”Registers a function to run before each request.The function will be called without any arguments.If the function returns a non-None value, it\’s handled asif it was the return value from the view and furtherrequest handling is stopped.\”\”\”self.before_request_funcs.setdefault(None, []).append(f)return f 从函数可以看到，这是一个装饰器,被该装饰器修饰的函数会添加到字典中。 5、调用正常的请求，返回一个该请求函数的值。 调用请求的源代码： 12345678910111213141516 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.requestif 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 automaticallyif getattr(rule, \’provide_automatic_options\’, False)    and req.method == \’OPTIONS\’:    return self.make_default_options_response()# otherwise dispatch to the handler for that endpointreturn self.view_functions[rule.endpoint](**req.view_args) 6、将请求函数返回值构造成响应类。 1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859 def make_response(self, rv):    \”\”\”Converts the return value from a view function to a real    response object that is an instance of :attr:response_class.The following types are allowed for `rv`:  ======================= ===========================================:attr:`response_class`  the object is returned unchanged:class:`str`            a response object is created with the                        string as body:class:`unicode`        a response object is created with the