基础概念

首先要了解 WSGI 规范的概念，WSGI(Web Server Gateway Interface)规范描述了web server(Gunicorn,uWSGI等)如何与web application(flask, django等)交互、web application如何处理请求，定义在 pep 3333。正是有了 WSGI 规范，我们才能在任意 web server 上跑各种 web 应用。WSGI API 定义看起来很简单:

def application(environ, start_response)
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• application 就是 WSGI app，一个可调用对象

• 参数:

  • environ: 一个包含 WSGI 环境信息的字典，由 WSGI 服务器提供，常见的 key 有 PATH_INFO，QUERY_STRING 等
  • start_response: 生成 WSGI 响应的回调函数，接收两个参数，status 和 headers

• 函数返回值为响应体的迭代器 ###简单举例 下面举个简单的例子，比如一个返回 hello world 的应用：

def application(environ, start_response):status = '200 OK'headers = [('Content-Type', 'text/html; charset=utf8')]start_response(status, headers)return  [b"<h1>Hello, World!</h1>"]
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werkzeug相关

werkzeug是Python实现的WSGI规范的使用函数库。 正如werkzeug官网Werkzeug上所说，werkzeug使用起来非常简单，但是却非常强大。关于使用简单的这个特性，官网给了一段示例代码。

from werkzeug.wrappers import Request, Response
@Request.application
def application(request):return Response('Hello World!')
if __name__ == '__main__':from werkzeug.serving import run_simplerun_simple('localhost', 4000, application)
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###简单小结 关于上面的代码我做一下总结: application--可调用对象,wsig模块中加括号括号执行 application的返回值--Response对象,wsgi中会对该对象加括号执行其__call__方法 一次成功的访问,由以下几步完成

1. 浏览器（client）发送一个请求（request
2. 服务器（server）接收到请求
3. 服务器处理请求
4. 返回处理的结果（response
5. 浏览器处理返回的结果，显示出来。

Detail

具体来说:

1. wigi相关模块通过建立socket拿到客户端发送的数据,然后进行解析,然后封装到environ中
2. web框架比如flask,他拿到environ,执行其内部各种调用函数,视图函数,然后返回Response对象
3. wigi相关模块拿到相应的Response对象,执行其__call__方法拿到app_iter对象,进行for循环进行socket.sendall(data)方法进行数据发送 ###源码 现在我们开始看一下源码:

def run_simple(hostname, port, application, use_reloader=False,use_debugger=False, use_evalex=True,extra_files=None, reloader_interval=1,reloader_type='auto', threaded=False,processes=1, request_handler=None, static_files=None,passthrough_errors=False, ssl_context=None):def log_startup(sock):display_hostname = hostname not in ('', '*') and hostname or 'localhost'if ':' in display_hostname:display_hostname = '[%s]' % display_hostnamequit_msg = '(Press CTRL+C to quit)'port = sock.getsockname()[1]_log('info', ' * Running on %s://%s:%d/ %s',ssl_context is None and 'http' or 'https',display_hostname, port, quit_msg)def inner():try:fd = int(os.environ['WERKZEUG_SERVER_FD'])except (LookupError, ValueError):fd = Nonesrv = make_server(hostname, port, application, threaded,processes, request_handler,passthrough_errors, ssl_context,fd=fd)if fd is None:log_startup(srv.socket)srv.serve_forever()inner()
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执行inner方法 然后执行make_server方法拿到其返回值并赋值给srv

def make_server(host=None, port=None, app=None, threaded=False, processes=1,request_handler=None, passthrough_errors=False,ssl_context=None, fd=None):"""Create a new server instance that is either threaded, or forksor just processes one request after another."""if threaded and processes > 1:raise ValueError("cannot have a multithreaded and ""multi process server.")elif threaded:return ThreadedWSGIServer(host, port, app, request_handler,passthrough_errors, ssl_context, fd=fd)elif processes > 1:return ForkingWSGIServer(host, port, app, processes, request_handler,passthrough_errors, ssl_context, fd=fd)else:return BaseWSGIServer(host, port, app, request_handler,passthrough_errors, ssl_context, fd=fd)
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以BaseWSGIServer类为例,将其实例化就是执行其__init__方法 因为类的各种继承,我就不一一细说了: 总的来说:

就是创建socket和定义处理request的类RequestHandleClass
其为:WSGIRequestHandler,自己看一下源码找一下吧
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然后执行srv.server_forver srv为BaseWSGIServer的实例,根据类的继承,去查找各种方法. 记住一点就是查找方法优先从自己的类定义中找,如果没有就去父类中找.时刻谨记self是谁

#BaseWSGIServer中定义def serve_forever(self):self.shutdown_signal = Falsetry:HTTPServer.serve_forever(self)except KeyboardInterrupt:passfinally:self.server_close()
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###BaseServerdef serve_forever(self, poll_interval=0.5):"""Handle one request at a time until shutdown.Polls for shutdown every poll_interval seconds. Ignoresself.timeout. If you need to do periodic tasks, do them inanother thread."""self.__is_shut_down.clear()try:# XXX: Consider using another file descriptor or connecting to the# socket to wake this up instead of polling. Polling reduces our# responsiveness to a shutdown request and wastes cpu at all other# times.with _ServerSelector() as selector:selector.register(self, selectors.EVENT_READ)while not self.__shutdown_request:ready = selector.select(poll_interval)if ready:self._handle_request_noblock()self.service_actions()finally:self.__shutdown_request = Falseself.__is_shut_down.set()
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# BaseServerdef _handle_request_noblock(self):"""Handle one request, without blocking.I assume that selector.select() has returned that the socket isreadable before this function was called, so there should be no risk ofblocking in get_request()."""try:request, client_address = self.get_request()except OSError:returnif self.verify_request(request, client_address):try:self.process_request(request, client_address)except Exception:self.handle_error(request, client_address)self.shutdown_request(request)except:self.shutdown_request(request)raiseelse:self.shutdown_request(request)
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执行process_request方法

    def process_request(self, request, client_address):"""Call finish_request.Overridden by ForkingMixIn and ThreadingMixIn."""self.finish_request(request, client_address)self.shutdown_request(request)
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Next

    def finish_request(self, request, client_address):"""Finish one request by instantiating RequestHandlerClass."""self.RequestHandlerClass(request, client_address, self)
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执行RequestHandlerClass类的实例化

执行BaseHTTPRequestHandler的handle方法 WSGIRequestHandler.handle_one_request

def handle_one_request(self):"""Handle a single HTTP request."""self.raw_requestline = self.rfile.readline()if not self.raw_requestline:self.close_connection = 1elif self.parse_request():return self.run_wsgi()
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    def run_wsgi(self):if self.headers.get('Expect', '').lower().strip() == '100-continue':self.wfile.write(b'HTTP/1.1 100 Continue\r\n\r\n')self.environ = environ = self.make_environ()headers_set = []headers_sent = []def write(data):assert headers_set, 'write() before start_response'if not headers_sent:status, response_headers = headers_sent[:] = headers_settry:code, msg = status.split(None, 1)except ValueError:code, msg = status, ""code = int(code)self.send_response(code, msg)header_keys = set()for key, value in response_headers:self.send_header(key, value)key = key.lower()header_keys.add(key)if not ('content-length' in header_keys orenviron['REQUEST_METHOD'] == 'HEAD' orcode < 200 or code in (204, 304)):self.close_connection = Trueself.send_header('Connection', 'close')if 'server' not in header_keys:self.send_header('Server', self.version_string())if 'date' not in header_keys:self.send_header('Date', self.date_time_string())self.end_headers()assert isinstance(data, bytes), 'applications must write bytes'self.wfile.write(data)self.wfile.flush()def start_response(status, response_headers, exc_info=None):if exc_info:try:if headers_sent:reraise(*exc_info)finally:exc_info = Noneelif headers_set:raise AssertionError('Headers already set')headers_set[:] = [status, response_headers]return writedef execute(app): # app_iter对象 包含了需要返回的各项数据application_iter = app(environ, start_response)  # Flask实例的call方法返回的的response对象的__call__方法返回的东西try:for data in application_iter:write(data)if not headers_sent:write(b'')finally:if hasattr(application_iter, 'close'):application_iter.close()application_iter = Nonetry:execute(self.server.app)except (socket.error, socket.timeout) as e:self.connection_dropped(e, environ)except Exception:if self.server.passthrough_errors:raisefrom werkzeug.debug.tbtools import get_current_tracebacktraceback = get_current_traceback(ignore_system_exceptions=True)try:# if we haven't yet sent the headers but they are set# we roll back to be able to set them again.if not headers_sent:del headers_set[:]execute(InternalServerError())except Exception:passself.server.log('error', 'Error on request:\n%s',traceback.plaintext)
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通过这个代码,我们拿到了app执行后拿到的可迭代对象 application_iter = app(environ, start_response) # Flask实例的call方法返回的的response对象的__call__方法返回的可迭代对象

END

最终for循环这个对象发送了数据

for data in application_iter:write(data)
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参考