Flask AppContext何时POP

首先请求进来走到

def wsgi_app(self, environ, start_response):
        """The actual WSGI application. This is not implemented in
        :meth:`__call__` so that middlewares can be applied without
        losing a reference to the app object. Instead of doing this::

            app = MyMiddleware(app)

        It's a better idea to do this instead::

            app.wsgi_app = MyMiddleware(app.wsgi_app)

        Then you still have the original application object around and
        can continue to call methods on it.

        .. versionchanged:: 0.7
            Teardown events for the request and app contexts are called
            even if an unhandled error occurs. Other events may not be
            called depending on when an error occurs during dispatch.
            See :ref:`callbacks-and-errors`.

        :param environ: A WSGI environment.
        :param start_response: A callable accepting a status code,
            a list of headers, and an optional exception context to
            start the 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:  # noqa: B001
                error = sys.exc_info()[1]
                raise
            return response(environ, start_response)
        finally:
            if self.should_ignore_error(error):
                error = None
            ctx.auto_pop(error)

auto_pop函数会调用request上下文pop，在里面会pop掉app上下文

    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)

 def pop(self, exc=_sentinel):
        """Pops the request context and unbinds it by doing that.  This will
        also trigger the execution of functions registered by the
        :meth:`~flask.Flask.teardown_request` decorator.

        .. versionchanged:: 0.9
           Added the `exc` argument.
        """
        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]
                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)  # pop 掉app ctx

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

这也就是为什么g对象挂载到app上下文下，其它请求过来也无法获取ｇ变量的数据