flask源码剖析之上下文全局变量

flask有四个线程级的全局变量，两个是程序上下文变量：current_app和g，另外两个是请求上下文变量：request和session.

这四个变量只有从flask.globals中导入才可以使用，即

from flask import current_app

from flask import g

from flask import request

from flask import session

但是使用不意味着有效，要想有效，导入后还要再push程序上下文变量和请求上下文变量：

1）利用flask.app中的app.app_context()或app.request_context()方法建立AppContext(app)实例或RequestContext(...)实例；

      app_ctx       = app.app_context()

      request_ctx = app.request_context(...)

    def app_context(self):        """Binds the application only.  For as long as the application is bound        to the current context the :data:`flask.current_app` points to that        application.  An application context is automatically created when a        request context is pushed if necessary.        Example usage::            with app.app_context():                ...        .. versionadded:: 0.9        """        return AppContext(self)    def request_context(self, environ):        """Creates a :class:`~flask.ctx.RequestContext` from the given        environment and binds it to the current context.  This must be used in        combination with the `with` statement because the request is only bound        to the current context for the duration of the `with` block.        Example usage::            with app.request_context(environ):                do_something_with(request)        The object returned can also be used without the `with` statement        which is useful for working in the shell.  The example above is        doing exactly the same as this code::            ctx = app.request_context(environ)            ctx.push()            try:                do_something_with(request)            finally:                ctx.pop()        .. versionchanged:: 0.3           Added support for non-with statement usage and `with` statement           is now passed the ctx object.        :param environ: a WSGI environment        """        return RequestContext(self, environ)

2）调用1）中建立的实例相应的push()方法——flask.ctx中；

      app_ctx.push( )

or   request_ctx.push( )    ???这个好像不需要？？？P12   

确实不需要，因为这是系统自动帮我们完成的！！！！具体参见文章Flask request，g，session的实现原理

class AppContext(object):    """The application context binds an application object implicitly    to the current thread or greenlet, similar to how the    :class:`RequestContext` binds request information.  The application    context is also implicitly created if a request context is created    but the application is not on top of the individual application    context.    """    def __init__(self, app):        self.app = app        self.url_adapter = app.create_url_adapter(None)        self.g = app.app_ctx_globals_class()        # Like request context, app contexts can be pushed multiple times        # but there a basic "refcount" is enough to track them.        self._refcnt = 0    def push(self):        """Binds the app context to the current context."""        self._refcnt += 1        _app_ctx_stack.push(self)        appcontext_pushed.send(self.app)    ......

3）2）中的push()方法实际上就是利用LocalStack()的实例_app_ctx_stack或_request_ctx_stack的push()方法——入栈。因此我们可以使用flask创建n个web app，而不会错乱。

一个web app有很多个request和session信息，使用_request_ctx_stack保存。

# -*- coding: utf-8 -*-"""    flask.globals    ~~~~~~~~~~~~~    Defines all the global objects that are proxies to the current    active context.    :copyright: (c) 2011 by Armin Ronacher.    :license: BSD, see LICENSE for more details."""from functools import partialfrom werkzeug.local import LocalStack, LocalProxydef _lookup_req_object(name):    top = _request_ctx_stack.top    if top is None:        raise RuntimeError('working outside of request context')    return getattr(top, name)def _lookup_app_object(name):    top = _app_ctx_stack.top    if top is None:        raise RuntimeError('working outside of application context')    return getattr(top, name)def _find_app():    top = _app_ctx_stack.top    if top is None:        raise RuntimeError('working outside of application context')    return top.app# context locals_request_ctx_stack = LocalStack()_app_ctx_stack = LocalStack()current_app = LocalProxy(_find_app)request = LocalProxy(partial(_lookup_req_object, 'request'))session = LocalProxy(partial(_lookup_req_object, 'session'))g = LocalProxy(partial(_lookup_app_object, 'g'))

4）因此，所谓的app_ctx.push()实质上是落实在LocalStack().push()即_app_ctx_stack.push()之上的！

可参考文章Flask request，g，session的实现原理