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 partial -
from werkzeug.local import LocalStack, LocalProxy -
def _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
出处：https://blog.csdn.net/LANGQING12345/article/details/46740551