Flask-信号（blinker）

简单了解信号

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。简单来说就是flask在列表里面，预留了几个空列表，在里面存东西。信号通过发送通知来帮助你解耦应用。简言之，信号允许某个发送者通知接收者有事情发生了；

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1	pip3 install blinker

一、 内置信号

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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')# 请求上下文执行完毕后自动执行（无论成功与否）   appcontext_pushed = _signals.signal('appcontext-pushed')            # 请求上下文push时执行 appcontext_popped = _signals.signal('appcontext-popped')            # 请求上下文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

                2. request_started = _signals.signal('request-started')                # 请求到来前执行
                5. request_finished = _signals.signal('request-finished')              # 请求结束后执行

                3. before_render_template = _signals.signal('before-render-template')  # 模板渲染前执行
                4. template_rendered = _signals.signal('template-rendered')            # 模板渲染后执行

                发生在2/3/4/5或不执行 got_request_exception = _signals.signal('got-request-exception')    # 请求执行出现异常时执行

                6. 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）
                7. appcontext_tearing_down = _signals.signal('appcontext-tearing-down')# 请求上下文执行完毕后自动执行（无论成功与否）

                1. appcontext_pushed = _signals.signal('appcontext-pushed')            # 请求app上下文push时执行

                8. appcontext_popped = _signals.signal('appcontext-popped')            # 请求上下文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

执行顺序

源码示例

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)
        response = self.make_response(rv)
        response = self.process_response(response)

        # ############### request_finished 信号 ###############
        request_finished.send(self, response=response)
        return response

    def wsgi_app(self, environ, start_response):

        ctx = self.request_context(environ)
        ctx.push()
        error = None
        try:
            try:
                response = self.full_dispatch_request()
            except Exception as e:
                error = e
                response = self.make_response(self.handle_exception(e))
            return response(environ, start_response)
        finally:
            if self.should_ignore_error(error):
                error = None
            ctx.auto_pop(error)

request_started

同上

request_finished

def render_template(template_name_or_list, **context):
    """Renders a template from the template folder with the given
    context.

    :param template_name_or_list: the name of the template to be
                                  rendered, or an iterable with template names
                                  the first one existing will be rendered
    :param context: the variables that should be available in the
                    context of the template.
    """
    ctx = _app_ctx_stack.top
    ctx.app.update_template_context(context)
    return _render(ctx.app.jinja_env.get_or_select_template(template_name_or_list),
                   context, ctx.app)

def _render(template, context, app):
    """Renders the template and fires the signal"""

    # ############### before_render_template 信号 ###############
    before_render_template.send(app, template=template, context=context)
    rv = template.render(context)

    # ############### template_rendered 信号 ###############
    template_rendered.send(app, template=template, context=context)
    return rv

before_render_template

同上

template_rendered

class Flask(_PackageBoundObject):

    def handle_exception(self, e):

        exc_type, exc_value, tb = sys.exc_info()

        # ############### got_request_exception 信号 ###############
        got_request_exception.send(self, exception=e)
        handler = self._find_error_handler(InternalServerError())

        if self.propagate_exceptions:
            # if we want to repropagate the exception, we can attempt to
            # raise it with the whole traceback in case we can do that
            # (the function was actually called from the except part)
            # otherwise, we just raise the error again
            if exc_value is e:
                reraise(exc_type, exc_value, tb)
            else:
                raise e

        self.log_exception((exc_type, exc_value, tb))
        if handler is None:
            return InternalServerError()
        return handler(e)

    def wsgi_app(self, environ, start_response):

        ctx = self.request_context(environ)
        ctx.push()
        error = None
        try:
            try:
                response = self.full_dispatch_request()
            except Exception as e:
                error = e
                # 这里这里这里这里这里这里这里这里这里这里这里这里 #
                response = self.make_response(self.handle_exception(e))
            return response(environ, start_response)
        finally:
            if self.should_ignore_error(error):
                error = None
            ctx.auto_pop(error)

got_request_exception

class AppContext(object):
    def push(self):
        """Binds the app context to the current context."""
        self._refcnt += 1
        if hasattr(sys, 'exc_clear'):
            sys.exc_clear()
        _app_ctx_stack.push(self)
        # ############## 触发 appcontext_pushed 信号 ##############
        appcontext_pushed.send(self.app)

    def pop(self, exc=_sentinel):
        """Pops the app context."""
        try:
            self._refcnt -= 1
            if self._refcnt <= 0:
                if exc is _sentinel:
                    exc = sys.exc_info()[1]
                # ############## 触发 appcontext_tearing_down 信号 ##############
                self.app.do_teardown_appcontext(exc)
        finally:
            rv = _app_ctx_stack.pop()
        assert rv is self, 'Popped wrong app context.  (%r instead of %r)' \
            % (rv, self)

        # ############## 触发 appcontext_popped 信号 ##############
        appcontext_popped.send(self.app)

class RequestContext(object):
    def push(self):
        top = _request_ctx_stack.top
        if top is not None and top.preserved:
            top.pop(top._preserved_exc)

        app_ctx = _app_ctx_stack.top
        if app_ctx is None or app_ctx.app != self.app:

            # ####################################################
            app_ctx = self.app.app_context()
            app_ctx.push()
            self._implicit_app_ctx_stack.append(app_ctx)
        else:
            self._implicit_app_ctx_stack.append(None)

        if hasattr(sys, 'exc_clear'):
            sys.exc_clear()

        _request_ctx_stack.push(self)

        # Open the session at the moment that the request context is
        # available. This allows a custom open_session method to use the
        # request context (e.g. code that access database information
        # stored on `g` instead of the appcontext).
        self.session = self.app.open_session(self.request)
        if self.session is None:
            self.session = self.app.make_null_session()

class Flask(_PackageBoundObject):

    def wsgi_app(self, environ, start_response):

        ctx = self.request_context(environ)
        ctx.push()
        error = None
        try:
            try:
                response = self.full_dispatch_request()
            except Exception as e:
                error = e
                response = self.make_response(self.handle_exception(e))
            return response(environ, start_response)
        finally:
            if self.should_ignore_error(error):
                error = None
            ctx.auto_pop(error)

    def pop(self, exc=_sentinel):
        app_ctx = self._implicit_app_ctx_stack.pop()

        try:
            clear_request = False
            if not self._implicit_app_ctx_stack:
                self.preserved = False
                self._preserved_exc = None
                if exc is _sentinel:
                    exc = sys.exc_info()[1]

                # ################## 触发 request_tearing_down 信号 ##################
                self.app.do_teardown_request(exc)

                # If this interpreter supports clearing the exception information
                # we do that now.  This will only go into effect on Python 2.x,
                # on 3.x it disappears automatically at the end of the exception
                # stack.
                if hasattr(sys, 'exc_clear'):
                    sys.exc_clear()

                request_close = getattr(self.request, 'close', None)
                if request_close is not None:
                    request_close()
                clear_request = True
        finally:
            rv = _request_ctx_stack.pop()

            # get rid of circular dependencies at the end of the request
            # so that we don't require the GC to be active.
            if clear_request:
                rv.request.environ['werkzeug.request'] = None

            # Get rid of the app as well if necessary.
            if app_ctx is not None:
                # ####################################################
                app_ctx.pop(exc)

            assert rv is self, 'Popped wrong request context.  ' \
                '(%r instead of %r)' % (rv, self)

    def auto_pop(self, exc):
        if self.request.environ.get('flask._preserve_context') or \
           (exc is not None and self.app.preserve_context_on_exception):
            self.preserved = True
            self._preserved_exc = exc
        else:
            self.pop(exc)

request_tearing_down

同上

appcontext_tearing_down

同上

appcontext_tearing_down

同上

appcontext_pushed

同上

appcontext_popped

def flash(message, category='message'):
    """Flashes a message to the next request.  In order to remove the
    flashed message from the session and to display it to the user,
    the template has to call :func:`get_flashed_messages`.

    .. versionchanged:: 0.3
       `category` parameter added.

    :param message: the message to be flashed.
    :param category: the category for the message.  The following values
                     are recommended: ``'message'`` for any kind of message,
                     ``'error'`` for errors, ``'info'`` for information
                     messages and ``'warning'`` for warnings.  However any
                     kind of string can be used as category.
    """
    # Original implementation:
    #
    #     session.setdefault('_flashes', []).append((category, message))
    #
    # This assumed that changes made to mutable structures in the session are
    # are always in sync with the session object, which is not true for session
    # implementations that use external storage for keeping their keys/values.
    flashes = session.get('_flashes', [])
    flashes.append((category, message))
    session['_flashes'] = flashes

    # ############### 触发 message_flashed 信号 ###############
    message_flashed.send(current_app._get_current_object(),
                         message=message, category=category)

message_flashed

二、 自定义信号

　第一步：创建信号

第二步：将函数注册到信号中： 添加到这个列表

　第三步： 发送信号

　第四步：运行

具体实现：可参考flask源码，写一个自定义信号

from flask import Flask,flash
from flask.signals import _signals
app = Flask(__name__)

xinhao = _signals.signal("xinhao")#创建信号
#定义函数
def wahaha(*args,**kwargs):
    print("",args,kwargs)

def sww(*args,**kwargs):
    print("",args,kwargs)
# 将函数注册到信号中,添加到这个列表
xinhao.connect(wahaha)
xinhao.connect(sww)

@app.route("/zzz")
def zzz():
    xinhao.send(sender='xxx',a1=123,a2=456)  #触发这个信号，执行注册到这个信号列表中的所有函数,此处的参数个数需要与定义的函数中的参数一致
    return "发送信号成功"

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

#打印结果
# 111 (None,) {'sender': 'xxx', 'a1': 123, 'a2': 456}
# 222 (None,) {'sender': 'xxx', 'a1': 123, 'a2': 456}

三、了解Django中的信号

点击查看

四、其他

1.chain的作用

v1 = [11,22,33,44]
v2 = [1,4,7,5]
from itertools import chain
ff = []
for i in chain(v1,v2):   #chain会把两个列表连接在一块
    ff.append(i)
print(ff)     #[11, 22, 33, 44, 1, 4, 7, 5]

2.特殊的装饰器（@app.before_first_request ；@app.before_request ； @app.after_request）和信号有什么区别？

　　　-  触发信号是没有返回值的，写不写返回值都无所谓

　　　-  特殊的装饰器对返回值是有意义的，当before_request有返回值时就不会执行后续视图函数了，没有返回值的时候才会执行后续函数，而after_request必须有返回值

　　       所以特殊装饰器的功能比信号的功能强大

3.信用用于做什么？

　　-自定义一些没有返回值的操作
　　-降低代码之间的耦合

4.通过信号可以做权限吗？

　　- 本身是做不了的，要想做得用其他的机制配合着来使用，这样做的话会闲的很麻烦，所以我们选择中间件来做