以前在博客园上写的文章，转到这里来。

这是一个最简单的demo：

from flask import Flaskapp = Flask(__name__)@app.route('/')def index():    return 'Hello, World!'if __name__ == '__main__':    app.run()

run()方法启动了应用，那么run()背后都调用哪些类、方法和函数呢？

将相关代码汇总起来，如下：

def run(self, host='localhost', port=5000, **options):        from werkzeug import run_simple        if 'debug' in options:            self.debug = options.pop('debug')        options.setdefault('use_reloader', self.debug)        options.setdefault('use_debugger', self.debug)        return run_simple(host, port, self, **options)def run_simple(hostname, port, application, use_reloader=False,               use_debugger=False, use_evalex=True,               extra_files=None, reloader_interval=1, threaded=False,               processes=1, request_handler=None, static_files=None,               passthrough_errors=False, ssl_context=None):    def inner():        make_server(hostname, port, application, threaded,                    processes, request_handler,                    passthrough_errors, ssl_context).serve_forever()def make_server(host, port, app=None, threaded=False, processes=1,                request_handler=None, passthrough_errors=False,                ssl_context=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)    elif processes > 1:        return ForkingWSGIServer(host, port, app, processes, request_handler,                                 passthrough_errors, ssl_context)    else:        return BaseWSGIServer(host, port, app, request_handler,                              passthrough_errors, ssl_context)class BaseWSGIServer(HTTPServer, object):    def __init__(self, host, port, app, handler=None,                 passthrough_errors=False, ssl_context=None):        if handler is None:            handler = WSGIRequestHandler        self.address_family = select_ip_version(host, port)        HTTPServer.__init__(self, (host, int(port)), handler)        self.app = app        self.passthrough_errors = passthrough_errors# BaseWSGIServer第四行class WSGIRequestHandler(BaseHTTPRequestHandler, object):    def run_wsgi(self):        app = self.server.app        environ = self.make_environ()        headers_set = []        headers_sent = []        def execute(app):            application_iter = app(environ, start_response)# 调用了__call__方法，所以要回到Flask类中寻找相关代码class Flask(object):    def __call__(self, environ, start_response):        return self.wsgi_app(environ, start_response)    def wsgi_app(self, environ, start_response):        with self.request_context(environ):            rv = self.preprocess_request()            if rv is None:                rv = self.dispatch_request()            response = self.make_response(rv)            response = self.process_response(response)            return response(environ, start_response)

使用graphviz将关系画出来，如下：