八、常用的装饰器

1、常用的装饰器有：before_request、after_request（这两个装饰器有点类似于django的中间件组件）

源码：

    def preprocess_request(self):
        """Called before the request is dispatched. Calls
        :attr:`url_value_preprocessors` registered with the app and the
        current blueprint (if any). Then calls :attr:`before_request_funcs`
        registered with the app and the blueprint.

        If any :meth:`before_request` handler returns a non-None value, the
        value is handled as if it was the return value from the view, and
        further request handling is stopped.
        """

        bp = _request_ctx_stack.top.request.blueprint

        funcs = self.url_value_preprocessors.get(None, ())
        if bp is not None and bp in self.url_value_preprocessors:
            funcs = chain(funcs, self.url_value_preprocessors[bp])
        for func in funcs:
            func(request.endpoint, request.view_args)

        funcs = self.before_request_funcs.get(None, ())
        if bp is not None and bp in self.before_request_funcs:
            funcs = chain(funcs, self.before_request_funcs[bp])
        for func in funcs:
24             rv = func()
25             if rv is not None:
26                 return rv

    def process_response(self, response):
        """Can be overridden in order to modify the response object
        before it's sent to the WSGI server.  By default this will
        call all the :meth:`after_request` decorated functions.

        .. versionchanged:: 0.5
           As of Flask 0.5 the functions registered for after request
           execution are called in reverse order of registration.

        :param response: a :attr:`response_class` object.
        :return: a new response object or the same, has to be an
                 instance of :attr:`response_class`.
        """
        ctx = _request_ctx_stack.top
        bp = ctx.request.blueprint
        funcs = ctx._after_request_functions
        if bp is not None and bp in self.after_request_funcs:
            funcs = chain(funcs, reversed(self.after_request_funcs[bp]))
        if None in self.after_request_funcs:
            funcs = chain(funcs, reversed(self.after_request_funcs[None]))
        for handler in funcs:
            response = handler(response)
        if not self.session_interface.is_null_session(ctx.session):
            self.session_interface.save_session(self, ctx.session, response)
        return response

练习：

from flask import Flask, Markup, request, redirect


app = Flask(__name__)


# app.before_first_request_funcs -> list
# 将被before_request装饰的函数添加到before_first_request_funcs列表中，在视图函数被执行前执行
# 如果某个被装饰的函数有返回值的，就会不在执行对应的视图函数，直接开始执行被after_request装饰器的函数(反向)


@app.before_request
def check_home():
    if "home" in request.path:
        return redirect("/")


@app.before_request
def usual_bye():
    print("Bye")


@app.before_request
def usual_input():
    print("Hello World!")


# app.after_request_funcs -> list
# 将被after_request装饰的函数添加到after_request_funcs列表中，在响应产生后被执行，函数必须要有一个接收response的参数，并且也要返回
# 一个响应
# 注意：after_request_funcs里面的函数执行的顺序是相反的

@app.after_request
def foo(response):
    print("foo")
    return response


@app.after_request
def bar(response):
    print("bar")
    return response


@app.route("/")
def index():
    return Markup("index|<a href='/home'>Home</a>")


@app.route("/home")
def home():
    return "home"


if __name__ == '__main__':
    app.run("localhost", 80, debug=True)

图示：

 

 2、其他装饰器：errorhandler、template_global、template_filter等

# 当发生相应的错误码时，执行对应的函数，返回一个界面良好的404界面
@app.errorhandler(404)
def show_404():
    from flask import render_template
    return render_template("404.html")


# 可以让所有的模板都使用的函数
@app.template_global
def gun(a, b):
    return a + b


# 可以让所有的模板中都使用的过滤器
@app.template_filter
def f(a, b):
    return a + b