flask 源码专题（三）：请求上下文和应用上下文入栈与出栈

# 将ctx入栈，但是内部也将应用上下文入栈
 ctx.push()

def push(self):
    # 获取到的  top  == ctx
    top = _request_ctx_stack.top
    if top is not None and top.preserved:
        top.pop(top._preserved_exc)
 
    # Before we push the request context we have to ensure that there
    # is an application context.
    """
        _request_ctx_stack 和 _app_ctx_stack 都是 Local 类的实例
    """
    # 获取 应用上下文的栈顶元素，得到 app_ctx
    app_ctx = _app_ctx_stack.top
    if app_ctx is None or app_ctx.app != self.app:
        # self.app == Fask()
        # 得到 一个 AppContext类的实例对象，得到一个 应用上下文对象 app_ctx，此时 app_ctx拥有以下属性: app_ctx.app = app, app_ctx.g = app.app_ctx_globals_class()
        app_ctx = self.app.app_context()
        # 将 app_ctx 入栈,应用上下文入栈
        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()
 
    # self 指的是 ctx，即将ctx入栈，即 _request_ctx_stack._local.stack = [ctx]。请求上下文入栈
    _request_ctx_stack.push(self)
    # 由于每次请求都会初始化创建你ctx，因此session都为None
    if self.session is None:
        # SecureCookieSessionInterface()
        # session_interface = SecureCookieSessionInterface(),即session_interface就是一个SecureCookieSessionInterface类的实例对象
        session_interface = self.app.session_interface
        # 第一次访问：生成一个 字典(容器) 返回至 self.session
        self.session = session_interface.open_session(
            self.app, self.request
        )
        if self.session is None:
            self.session = session_interface.make_null_session(self.app)

class AppContext(object):
　　 def push(self):
        _app_ctx_stack.push(self)     #把新创建的app_ctx上下文app对象添加到了_app_ctx_stack这个栈中
        appcontext_pushed.send(self.app)   #在这里遇到了第一个信号，请求app上下文push时执行

    def __init__(self):
        self._local = Local()

class Local(object):
    __slots__ = (‘__storage__‘, ‘__ident_func__‘)
 
    def __init__(self):
        object.__setattr__(self, ‘__storage__‘, {})
        object.__setattr__(self, ‘__ident_func__‘, get_ident)

"""
    注意：
        在项目启动之后，global里的代码就已经执行完毕，而且也只会执行一次，因此这里面的变量是针对所有请求所使用的，但是根据不同线程id用来存放各自的值
"""
#  生成 请求上下文栈对象，将请求上下文对象 ctx 保存到 _request_ctx_stack._local.stack = [ctx]中
_request_ctx_stack = LocalStack()
# 生成应用上下文栈对象，将应用上下文对象 app_ctx 保存到  _app_ctx_stack._local.stack = [app_ctx]中
_app_ctx_stack = LocalStack()
  
# current_app.__local = app
current_app = LocalProxy(_find_app)
# 获取ctx.request
request = LocalProxy(partial(_lookup_req_object, ‘request‘))
# 获取 ctx.session
session = LocalProxy(partial(_lookup_req_object, ‘session‘))
# 维护此次请求的一个全局变量，其实就是一个字典
g = LocalProxy(partial(_lookup_app_object, ‘g‘))

def push(self, obj):
    # obj == ctx
    """Pushes a new item to the stack"""
    rv = getattr(self._local, ‘stack‘, None)
    if rv is None:
        self._local.stack = rv = []
    rv.append(obj)
    return rv

def __setattr__(self, name, value):
        ident = self.__ident_func__()
        storage = self.__storage__
        try:
            storage[ident][name] = valueexcept KeyError:
            storage[ident] = {name: value}

_request_ctx_stack._local.stack={
        线程id或协程id: {
            ‘stack‘: [ctx]
        }
}  

# 出栈，删除本次请求的相关信息
ctx.auto_pop(error)

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:
        # 请求上下文出栈
        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:
            # 应用上下文出栈
            app_ctx.pop(exc)
 
        assert rv is self, ‘Popped wrong request context.  ‘             ‘(%r instead of %r)‘ % (rv, self)

def pop(self):
    """Removes the topmost item from the stack, will return the
    old value or `None` if the stack was already empty.
    """
    stack = getattr(self._local, ‘stack‘, None)
    if stack is None:
        return None
    elif len(stack) == 1:
        release_local(self._local)
        return stack[-1]
    else:
        # 获取并删除列表中的第一个元素，同时返回该元素
        return stack.pop()

def release_local(local):
     
    local.__release_local__()

def __release_local__(self):
    # 将 self.__storage__ 所维护的字典中删除当前协程或线程id为key的元素
    self.__storage__.pop(self.__ident_func__(), None)