Flask源码：Signals.py（Flask信号量定义）

"""
    flask.signals
    ~~~~~~~~~~~~~

    Implements signals based on blinker if available, otherwise
    falls silently back to a noop.
    #定义了Flask信号，利用假类保持接口不变

    :copyright: 2010 Pallets
    :license: BSD-3-Clause
"""
try:
    from blinker import Namespace

    signals_available = True
except ImportError:
    signals_available = False

    class Namespace(object):
        def signal(self, name, doc=None):
            return _FakeSignal(name, doc)
    
    #创建假类处理导入异常
    class _FakeSignal(object):
        """If blinker is unavailable, create a fake class with the same
        interface that allows sending of signals but will fail with an
        error on anything else.  Instead of doing anything on send, it
        will just ignore the arguments and do nothing instead.
        """

        def __init__(self, name, doc=None):
            self.name = name
            self.__doc__ = doc

        def send(self, *args, **kwargs):
            pass

        def _fail(self, *args, **kwargs):
            raise RuntimeError(
                "Signalling support is unavailable because the blinker"
                " library is not installed."
            )
        
        #将原接口函数替换
        connect = connect_via = connected_to = temporarily_connected_to = _fail
        disconnect = _fail
        has_receivers_for = receivers_for = _fail
        #隐藏_fail函数
        del _fail

_signals = Namespace()

#定义一系列Flaks使用的信号
template_rendered = _signals.signal("template-rendered")
before_render_template = _signals.signal("before-render-template")
request_started = _signals.signal("request-started")
request_finished = _signals.signal("request-finished")
request_tearing_down = _signals.signal("request-tearing-down")
got_request_exception = _signals.signal("got-request-exception")
appcontext_tearing_down = _signals.signal("appcontext-tearing-down")
appcontext_pushed = _signals.signal("appcontext-pushed")
appcontext_popped = _signals.signal("appcontext-popped")
message_flashed = _signals.signal("message-flashed")