Flask本身没有信号，需要安装第三方组件。Flask框架中的信号基于blinker，其主要就是让开发者可是在flask请求过程中定制一些用户行为。

pip3 install blinker

一、内置信号

request_started = _signals.signal('request-started')                # 请求到来前执行request_finished = _signals.signal('request-finished')              # 请求结束后执行 before_render_template = _signals.signal('before-render-template')  # 模板渲染前执行template_rendered = _signals.signal('template-rendered')            # 模板渲染后执行 got_request_exception = _signals.signal('got-request-exception')    # 请求执行出现异常时执行 request_tearing_down = _signals.signal('request-tearing-down')      # 请求执行完毕后自动执行（无论成功与否）appcontext_tearing_down = _signals.signal('appcontext-tearing-down')# 应用上下文执行完毕后自动执行（无论成功与否） appcontext_pushed = _signals.signal('appcontext-pushed')            # 应用上下文push时执行appcontext_popped = _signals.signal('appcontext-popped')            # 应用上下文pop时执行message_flashed = _signals.signal('message-flashed')                # 调用flask在其中添加数据时，自动触发

部分源码流程：

class Flask(_PackageBoundObject):    def full_dispatch_request(self):        self.try_trigger_before_first_request_functions()        try:            ########## 触发request_started信号 ##########            request_started.send(self)                   rv = self.preprocess_request()            if rv is None:                rv = self.dispatch_request()        except Exception as e:            rv = self.handle_user_exception(e)        return self.finalize_request(rv)    def wsgi_app(self, environ, start_response):        ctx = self.request_context(environ)        error = None        try:            try:                ctx.push()                response = self.full_dispatch_request()            except Exception as e:                error = e                response = self.handle_exception(e)            except:                error = sys.exc_info()[1]                raise            return response(environ, start_response)        finally:            if self.should_ignore_error(error):                error = None            ctx.auto_pop(error)

class Flask(_PackageBoundObject):    def finalize_request(self, rv, from_error_handler=False):    response = self.make_response(rv)    try:        response = self.process_response(response)        ########## request_finished信号 ##########        request_finished.send(self, response=response)    except Exception:        if not from_error_handler:            raise        self.logger.exception('Request finalizing failed with an error while handling an error')    return response

二、自定义信号

from flask import Flask, current_app, flash, render_templatefrom flask.signals import _signalsapp = Flask(import_name=__name__)# 自定义信号xx = _signals.signal("xx")def func(sender, *args, **kwargs):    print(sender, args, kwargs)  # 123 () {'k1': 'v1'}# 自定义信号中注册函数xx.connect(func)@app.route("/index")def index():    # 触发信号    xx.send("123", k1="v1")    return "index"if __name__ == "__main__":    app.run()