前言：

Flask是目前为止我最喜欢的一个Python Web框架了，为了更好的掌握其内部实现机制，这两天准备学习下Flask的源码，将由浅入深跟大家分享下，其中Flask版本为1.1.1。

Flask系列文章：

1. Flask开发初探

正文

本文将结合源码跟踪看下Flask是如何启动并运行一个服务的。

首先，继续贴上最简单的应用：

from flask import Flask
app = Flask(__name__)

@app.route('/')
def hello_world():
    return 'Hello Flask!'

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

我们看到，这段代码先初始化了Flask类并被app所指向，然后执行run()来启动程序的。

查看run方法：

def run(self, host=None, port=None, debug=None, load_dotenv=True, **options):
        if os.environ.get("FLASK_RUN_FROM_CLI") == "true":
            from .debughelpers import explain_ignored_app_run

            explain_ignored_app_run()
            return

        if get_load_dotenv(load_dotenv):
            cli.load_dotenv()

            # if set, let env vars override previous values
            if "FLASK_ENV" in os.environ:
                self.env = get_env()
                self.debug = get_debug_flag()
            elif "FLASK_DEBUG" in os.environ:
                self.debug = get_debug_flag()

        # debug passed to method overrides all other sources
        if debug is not None:
            self.debug = bool(debug)

        _host = "127.0.0.1"
        _port = 5000
        server_name = self.config.get("SERVER_NAME")
        sn_host, sn_port = None, None

        if server_name:
            sn_host, _, sn_port = server_name.partition(":")

        host = host or sn_host or _host
        # pick the first value that's not None (0 is allowed)
        port = int(next((p for p in (port, sn_port) if p is not None), _port))

        options.setdefault("use_reloader", self.debug)
        options.setdefault("use_debugger", self.debug)
        options.setdefault("threaded", True)

        cli.show_server_banner(self.env, self.debug, self.name, False)

        from werkzeug.serving import run_simple

        try:
            run_simple(host, port, self, **options)
        finally:
            # reset the first request information if the development server
            # reset normally.  This makes it possible to restart the server
            # without reloader and that stuff from an interactive shell.
            self._got_first_request = False

首先入参：

该方法的处理流程是：对入参进行配置处理之后，执行werkzeug的run_simple()方法，

run_simple将启动一个WSGI服务。

关于WSGI协议：

1. 它是关于HTTP服务器和Web应用的桥梁，定义了标准接口以提升Web应用之间的可移植性，是一套接口交互规范。
2. 它的功能是监听指定端口服务，将来自HTTP服务器的请求解析为WSGI格式，调用Flask app处理请求。

run_simple中的inner方法是核心，inner调用make_server().serve_forever()启动服务。关于make_server:

def make_server(host=None, port=None, app=None, threaded=False, processes=1,
                request_handler=None, passthrough_errors=False,
                ssl_context=None, fd=None):
    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)

make_server会根据线程或者进程数返回相应的WSGI服务器，默认情况下返回BaseWSGIServer，ThreadedWSGIServer和ForkingWSGIServer均集成了BaserWSGIServer，接下来我们看下serve_forever()方法：

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

最终调用了Python标准类库接口HTTPServer的serve_forever()方法，而HTTPServer又是socketserver.TCPServer的子类，通过server_bind来监听服务：

class HTTPServer(socketserver.TCPServer):

    allow_reuse_address = 1    # Seems to make sense in testing environment

    def server_bind(self):
        """Override server_bind to store the server name."""
        socketserver.TCPServer.server_bind(self)
        host, port = self.server_address[:2]
        self.server_name = socket.getfqdn(host)
        self.server_port = port

以上，就是Flask服务启动的流程。