Flask中全局变量的实现

我们都知道在Flask中g，request，session和request是作为全局对象来提供信息的，既然是全局的又如何保持线程安全呢，接下来我们就看看flask是如何做到这点的。在源码中的ctx.py中有AppContext和RequestContext两个类,他们分别管理应用上下文和请求上下文.两者的实现也差不多,这里我们看看AppContext的实现

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
        if hasattr(sys, 'exc_clear'):
            sys.exc_clear()
        _app_ctx_stack.push(self)
        appcontext_pushed.send(self.app)

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

    def __enter__(self):
        self.push()
        return self

    def __exit__(self, exc_type, exc_value, tb):
        self.pop(exc_value)

        if BROKEN_PYPY_CTXMGR_EXIT and exc_type is not None:
            reraise(exc_type, exc_value, tb)

可以看到初始化中取得app,_refcnt记录了推入栈的上下文的数目,该类有push方法和pop方法推入和弹出app的上下文,并发送信号通知,关于信号可看http://www.jb51.net/article/59286.htm。do_teardown_appcontext就是运行注册的应用结束要运行的函数,然后的__enter__和__exit__是实现with的魔法方法.下面就是app_context_stack的实现

def _lookup_app_object(name):
    top = _app_ctx_stack.top
    if top is None:
        raise RuntimeError(_app_ctx_err_msg)
    return getattr(top, name)

def _find_app():
    top = _app_ctx_stack.top
    if top is None:
        raise RuntimeError(_app_ctx_err_msg)
    return top.app

_app_ctx_stack = LocalStack()
current_app = LocalProxy(_find_app)

可以看到这个栈是LocalStack的实例,在werkzeug.local可以看到它的实现

class Local(object):
    __slots__ = ('__storage__', '__ident_func__')

    def __init__(self):
        object.__setattr__(self, '__storage__', {})
        object.__setattr__(self, '__ident_func__', get_ident)

    def __iter__(self):
        return iter(self.__storage__.items())

    def __call__(self, proxy):
        """Create a proxy for a name."""
        return LocalProxy(self, proxy)

    def __release_local__(self):
        self.__storage__.pop(self.__ident_func__(), None)

    def __getattr__(self, name):
        try:
            return self.__storage__[self.__ident_func__()][name]
        except KeyError:
            raise AttributeError(name)

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

    def __delattr__(self, name):
        try:
            del self.__storage__[self.__ident_func__()][name]
        except KeyError:
            raise AttributeError(name)

其中get_ident是得到当前的线程号,Local通过以线程号作为Key来建立字典,以保证线程间的隔离,这个字典的每个Value也是个字典,用来设置当时线程中的属性.

最后,我们再看看这个

class LocalStack(object):

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

    def __release_local__(self):
        self._local.__release_local__()

    def _get__ident_func__(self):
        return self._local.__ident_func__

    def _set__ident_func__(self, value):
        object.__setattr__(self._local, '__ident_func__', value)
    __ident_func__ = property(_get__ident_func__, _set__ident_func__)
    del _get__ident_func__, _set__ident_func__

    def __call__(self):
        def _lookup():
            rv = self.top
            if rv is None:
                raise RuntimeError('object unbound')
            return rv
        return LocalProxy(_lookup)

    def push(self, obj):
        """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 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:
            return stack.pop()

    @property
    def top(self):
        """The topmost item on the stack.  If the stack is empty,
        `None` is returned.
        """
        try:
            return self._local.stack[-1]
        except (AttributeError, IndexError):
            return None

Local_Stack是通过Local来保证线程安全的基础上,加入栈的功能,至此这个栈就实现了

还有一点就是Local_Proxy的功能,他是作为LocalStack和Local的代理使用的,所有发给LocalProxy的请求以及对localStack和Local的调用都会交由LocalStack将请求转发给适当的被代理对象处理

class LocalProxy(object):

    """Acts as a proxy for a werkzeug local.  Forwards all operations to
    a proxied object.  The only operations not supported for forwarding
    are right handed operands and any kind of assignment.

    Example usage::

        from werkzeug.local import Local
        l = Local()

        # these are proxies
        request = l('request')
        user = l('user')

        from werkzeug.local import LocalStack
        _response_local = LocalStack()

        # this is a proxy
        response = _response_local()

    Whenever something is bound to l.user / l.request the proxy objects
    will forward all operations.  If no object is bound a :exc:`RuntimeError`
    will be raised.

    To create proxies to :class:`Local` or :class:`LocalStack` objects,
    call the object as shown above.  If you want to have a proxy to an
    object looked up by a function, you can (as of Werkzeug 0.6.1) pass
    a function to the :class:`LocalProxy` constructor::

        session = LocalProxy(lambda: get_current_request().session)

    .. versionchanged:: 0.6.1
       The class can be instanciated with a callable as well now.
    """
    __slots__ = ('__local', '__dict__', '__name__')

    def __init__(self, local, name=None):
        object.__setattr__(self, '_LocalProxy__local', local)
        object.__setattr__(self, '__name__', name)

    def _get_current_object(self):
        """Return the current object.  This is useful if you want the real
        object behind the proxy at a time for performance reasons or because
        you want to pass the object into a different context.
        """
        if not hasattr(self.__local, '__release_local__'):
            return self.__local()
        try:
            return getattr(self.__local, self.__name__)
        except AttributeError:
            raise RuntimeError('no object bound to %s' % self.__name__)

    @property
    def __dict__(self):
        try:
            return self._get_current_object().__dict__
        except RuntimeError:
            raise AttributeError('__dict__')

    def __repr__(self):
        try:
            obj = self._get_current_object()
        except RuntimeError:
            return '<%s unbound>' % self.__class__.__name__
        return repr(obj)

    def __bool__(self):
        try:
            return bool(self._get_current_object())
        except RuntimeError:
            return False

    def __unicode__(self):
        try:
            return unicode(self._get_current_object())  # noqa
        except RuntimeError:
            return repr(self)

    def __dir__(self):
        try:
            return dir(self._get_current_object())
        except RuntimeError:
            return []

    def __getattr__(self, name):
        if name == '__members__':
            return dir(self._get_current_object())
        return getattr(self._get_current_object(), name)

    def __setitem__(self, key, value):
        self._get_current_object()[key] = value

    def __delitem__(self, key):
        del self._get_current_object()[key]

    if PY2:
        __getslice__ = lambda x, i, j: x._get_current_object()[i:j]

        def __setslice__(self, i, j, seq):
            self._get_current_object()[i:j] = seq

        def __delslice__(self, i, j):
            del self._get_current_object()[i:j]

    __setattr__ = lambda x, n, v: setattr(x._get_current_object(), n, v)
    __delattr__ = lambda x, n: delattr(x._get_current_object(), n)
    __str__ = lambda x: str(x._get_current_object())
    __lt__ = lambda x, o: x._get_current_object() < o
    __le__ = lambda x, o: x._get_current_object() <= o
    __eq__ = lambda x, o: x._get_current_object() == o
    __ne__ = lambda x, o: x._get_current_object() != o
    __gt__ = lambda x, o: x._get_current_object() > o
    __ge__ = lambda x, o: x._get_current_object() >= o
    __cmp__ = lambda x, o: cmp(x._get_current_object(), o)  # noqa
    __hash__ = lambda x: hash(x._get_current_object())
    __call__ = lambda x, *a, **kw: x._get_current_object()(*a, **kw)
    __len__ = lambda x: len(x._get_current_object())
    __getitem__ = lambda x, i: x._get_current_object()[i]
    __iter__ = lambda x: iter(x._get_current_object())
    __contains__ = lambda x, i: i in x._get_current_object()
    __add__ = lambda x, o: x._get_current_object() + o
    __sub__ = lambda x, o: x._get_current_object() - o
    __mul__ = lambda x, o: x._get_current_object() * o
    __floordiv__ = lambda x, o: x._get_current_object() // o
    __mod__ = lambda x, o: x._get_current_object() % o
    __divmod__ = lambda x, o: x._get_current_object().__divmod__(o)
    __pow__ = lambda x, o: x._get_current_object() ** o
    __lshift__ = lambda x, o: x._get_current_object() << o
    __rshift__ = lambda x, o: x._get_current_object() >> o
    __and__ = lambda x, o: x._get_current_object() & o
    __xor__ = lambda x, o: x._get_current_object() ^ o
    __or__ = lambda x, o: x._get_current_object() | o
    __div__ = lambda x, o: x._get_current_object().__div__(o)
    __truediv__ = lambda x, o: x._get_current_object().__truediv__(o)
    __neg__ = lambda x: -(x._get_current_object())
    __pos__ = lambda x: +(x._get_current_object())
    __abs__ = lambda x: abs(x._get_current_object())
    __invert__ = lambda x: ~(x._get_current_object())
    __complex__ = lambda x: complex(x._get_current_object())
    __int__ = lambda x: int(x._get_current_object())
    __long__ = lambda x: long(x._get_current_object())  # noqa
    __float__ = lambda x: float(x._get_current_object())
    __oct__ = lambda x: oct(x._get_current_object())
    __hex__ = lambda x: hex(x._get_current_object())
    __index__ = lambda x: x._get_current_object().__index__()
    __coerce__ = lambda x, o: x._get_current_object().__coerce__(x, o)
    __enter__ = lambda x: x._get_current_object().__enter__()
    __exit__ = lambda x, *a, **kw: x._get_current_object().__exit__(*a, **kw)
    __radd__ = lambda x, o: o + x._get_current_object()
    __rsub__ = lambda x, o: o - x._get_current_object()
    __rmul__ = lambda x, o: o * x._get_current_object()
    __rdiv__ = lambda x, o: o / x._get_current_object()
    if PY2:
        __rtruediv__ = lambda x, o: x._get_current_object().__rtruediv__(o)
    else:
        __rtruediv__ = __rdiv__
    __rfloordiv__ = lambda x, o: o // x._get_current_object()
    __rmod__ = lambda x, o: o % x._get_current_object()
    __rdivmod__ = lambda x, o: x._get_current_object().__rdivmod__(o)

这就是AppContext基本的实现,RequestContext的实现也差不多,只是添加了异常处理可以在一定条件下不讲requestcontext弹出来以便将来的异常分析