Flask源码分析（一）Flask是如何运行起来的

app.run（）的执行流程

1.首先来看最简单的一个flask应用。app是Flask类的实例，最后执行了app的run方法。

from flask import Flask

app = Flask(__name__)

@app.route('/')
def index():
    return "hello world!"

if __name__ == '__main__':
    app.run()

2.来看一下run方法的执行过程。可以看到，run实际是执行了werkzeug.serving的run_simple函数（无关源码已删减），关键注意这里是传入了self，代表传入了当前Flask类的实例。然后继续查看run_simple，run_simple实际是执行了其内部定义的inner函数。

def run(self, host=None, port=None, debug=None, load_dotenv=True, **options):
    from werkzeug.serving import run_simple
    try:
        run_simple(host, port, self, **options)
    finally:
        self._got_first_request = False

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 inner():
        try:
            fd = int(os.environ["WERKZEUG_SERVER_FD"])
        except (LookupError, ValueError):
            fd = None
        srv = 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()

    if use_reloader:
        run_with_reloader(inner, extra_files, reloader_interval, reloader_type)
    else:
        inner()

3.inner函数调用make_server函数，注意这里传入的主要的4个值：hostname、port、application（即Flask实例）、request_handler（None值）。make_server函数实际是返回BaseWSGIServer的实例然后执行其内部的serve_forrever方法。这里的BaseWSGIServer即一个本地的server程序，默认是单进程单线程的（但是使用了IO多路复用技术，后续会提到）。

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 forks
    or 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
        )

4.重点来了！来看BaseWSGIServer源码。注意，这里涉及到多个类继承，关系有点复杂。首先看方法的调用关系。serve_forever方法是执行的HTTPServer的serve_forever方法，而HTTPServer.serve_forever是调用自身的 _handle_request_noblock方法（这里使用的IO多路复用技术），而_handle_request_noblock方法调用自身的process_request方法来接收请求，process_request将收到的请求交由finish_request方法处理。最终，交给RequestHandlerClass处理。

这里注意传入的三个参数：request、client_address、self。self是BaseWSGIServer自身的实例。

而这个ReuqustHandlerClass是什么？看BaseWSGIServer类__init__方法中这句代码：HTTPServer.__init__(self, server_address, handler)，就知道，ReuqustHandlerClass即是WSGIRequestHandler这个类。

这里总结成一句话，就是请求到来时，会将请求传给WSGIRequestHandler这个类，进行实例化。而相应处理请求呢？肯定在WSGIRequestHandler类中的__init__方法中啊！

class BaseWSGIServer(HTTPServer, object):

    """Simple single-threaded, single-process WSGI server."""

    def __init__(
        self,
        host,
        port,
        app,
        handler=None,
        passthrough_errors=False,
        ssl_context=None,
        fd=None,
    ):
        if handler is None:
            handler = WSGIRequestHandler

        HTTPServer.__init__(self, server_address, handler)

        self.app = app

    def serve_forever(self):
        self.shutdown_signal = False
        try:
            HTTPServer.serve_forever(self)
        except KeyboardInterrupt:
            pass
        finally:
            self.server_close()


class HTTPServer(socketserver.TCPServer):
    pass


class TCPServer(BaseServer):
    def __init__(self, server_address, RequestHandlerClass, bind_and_activate=True):
        """Constructor.  May be extended, do not override."""
        BaseServer.__init__(self, server_address, RequestHandlerClass)


class BaseServer:
    def __init__(self, server_address, RequestHandlerClass):
        """Constructor.  May be extended, do not override."""
        self.server_address = server_address
        self.RequestHandlerClass = RequestHandlerClass
        self.__is_shut_down = threading.Event()
        self.__shutdown_request = False


    def serve_forever(self, poll_interval=0.5):
        try:
            with _ServerSelector() as selector:
                selector.register(self, selectors.EVENT_READ)

                while not self.__shutdown_request:
                    ready = selector.select(poll_interval)
                    # bpo-35017: shutdown() called during select(), exit immediately.
                    if self.__shutdown_request:
                        break
                    if ready:
                        self._handle_request_noblock()

                    self.service_actions()
        finally:
            self.__shutdown_request = False
            self.__is_shut_down.set()


    def _handle_request_noblock(self):
        try:
            request, client_address = self.get_request()
        except OSError:
            return
        if 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)
                raise
        else:
            self.shutdown_request(request)


    def process_request(self, request, client_address):
        self.finish_request(request, client_address)
        self.shutdown_request(request)


    def finish_request(self, request, client_address):
        """Finish one request by instantiating RequestHandlerClass."""
        self.RequestHandlerClass(request, client_address, self)

5.接着看WSGIRequestHandler这个类的代码，这里又涉及到复杂的继承关系。首先看WSGIRequestHandler的实例化方法，在其父类BaseRequestHandler中，找到了实例化方法，果然，是在实例化方法时，执行自身的handle方法，来处理请求。

这里注意实例化传入的三个参数：request、client_address、server。server对应上面提到的BaseWSGIServer自身的实例。

注意，这里有点绕，仔细看。当请求到来时，执行handle方法，而handle方法已被WSGIRequestHandler重写，重写后的handle方法执行BaseHTTPRequestHandler的handle方法，实际是执行handle_one_request方法，而handle_one_request又被WSGIRequestHandler重写，重写后的handle_one_request实际是执行自身的run_wsgi方法。所以，请求的最终处理就在这里。

run_wsgi方法实际是执行内部定义的execute方法，此时传入的是self.server.app。self.server对应的是BaseWSGIServer的实例，而.app就是一开始run_simple(host, port, self, **options)传入的self，即Flask实例。而execute方法中执行app(environ, start_response)，是对象（）的形式。这里会调用Flask实例的__call__方法，又回到最开始的地方了。

所以，最终app.run（）所执行的流程，都在这里了。下一节我会继续分享，请求的处理过程。

class WSGIRequestHandler(BaseHTTPRequestHandler, object):

    def run_wsgi(self):
        """处理request"""
        def write(data):
            pass

        def start_response(status, response_headers, exc_info=None):
            pass

        def execute(app):
            application_iter = app(environ, start_response)
            try:
                for data in application_iter:
                    write(data)
                if not headers_sent:
                    write(b"")
            finally:
                if hasattr(application_iter, "close"):
                    application_iter.close()

        try:
            execute(self.server.app)
        except Exception:
            """错误处理"""
            pass

    def handle(self):
        try:
            BaseHTTPRequestHandler.handle(self)
        except (_ConnectionError, socket.timeout) as e:
            self.connection_dropped(e)
        except Exception as e:
            if self.server.ssl_context is None or not is_ssl_error(e):
                raise
        if self.server.shutdown_signal:
            self.initiate_shutdown()


    def handle_one_request(self):
        self.raw_requestline = self.rfile.readline()
        if not self.raw_requestline:
            self.close_connection = 1
        elif self.parse_request():
            return self.run_wsgi()


class BaseHTTPRequestHandler(socketserver.StreamRequestHandler):
    def handle_one_request(self):
        pass

    def handle(self):
        self.close_connection = True

        self.handle_one_request()
        while not self.close_connection:
            self.handle_one_request()


class StreamRequestHandler(BaseRequestHandler):
    pass


class BaseRequestHandler:

    def __init__(self, request, client_address, server):
        self.request = request
        self.client_address = client_address
        self.server = server
        self.setup()
        try:
            self.handle()
        finally:
            self.finish()

    def handle(self):
        pass

class Flask(_PackageBoundObject):
    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)