def open_url(url:
                ^
    SyntaxError: invalid syntax
    原来是忘了写')'。。。。)

class Local(object):
    ## request对象是Local的实例
    __slots__ = ('__storage__', '__ident_func__')
    def __init__(self):
        ## object.__setattr__如此赋值，并不调用实例的__setattr__方法
        object.__setattr__(self, '__storage__', {})
        ## 这里的__ident_func__存的的是当前进程／协程的id
        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:
            ## 每次调用request就是调用当前进程／协程下的request
            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)
    通过阅读flask的内部实现就明白了到底是如何优雅的实现：使用这些全局变量的时候，你啥都不用管只要调用就行了。这就是python的优雅的一 面。而的代码就是类似于这种的优雅的实现，多多‘临摹’高手的代码，可以学到更多优雅技巧：装饰器、协程、生成器、魔法方法等。而不是光学会概念、写一些例子。阅读代码中看到实际的应用代码片段，更加有助于自己以后用到自己的代码中。
    阅读代码有助于养成的代码风格
    “的代码不需要文档”，这句话虽然说的有些夸张的成份，但是也并无一定道理。的项目中的代码，注释占的比重是相当大的。比方tornado框架中的代码：
    class HTTPServer(TCPServer, Configurable,
                 httputil.HTTPServerConnectionDelegate):
    r"""A non-blocking, single-threaded HTTP server.
    A server is defined by a subclass of `.HTTPServerConnectionDelegate`,
    or, for backwards compatibility, a callback that takes an
    `.HTTPServerRequest` as an argument. The delegate is usually a
    `tornado.web.Application`.
    `HTTPServer` supports keep-alive connections by default
    (automatically for HTTP/1.1, or for HTTP/1.0 when the client
    requests ``Connection: keep-alive``).
    If ``xheaders`` is ``True``, we support the
    ``X-Real-Ip``/``X-Forwarded-For`` and
    ``X-Scheme``/``X-Forwarded-Proto`` headers, which override the
    remote IP and URI scheme/protocol for all requests.  These headers
    are useful when running Tornado behind a reverse proxy or load
    balancer.  The ``protocol`` argument can also be set to ``https``
    if Tornado is run behind an SSL-decoding proxy that does not set one of
    the supported ``xheaders``.
    To make this server serve SSL traffic, send the ``ssl_options`` keyword
    argument with an `ssl.SSLContext` object. For compatibility with older
    versions of Python ``ssl_options`` may also be a dictionary of keyword
    arguments for the `ssl.wrap_socket` method.::
       ssl_ctx = ssl.create_default_context(ssl.Purpose.CLIENT_AUTH)
       ssl_ctx.load_cert_chain(os.path.join(data_dir, "mydomain.crt"),
                               os.path.join(data_dir, "mydomain.key"))
       HTTPServer(applicaton, ssl_options=ssl_ctx)
    `HTTPServer` initialization follows one of three patterns (the
    initialization methods are defined on `tornado.tcpserver.TCPServer`):
    1. `~tornado.tcpserver.TCPServer.listen`: simple single-process::
            server = HTTPServer(app)
            server.listen(8888)
            IOLoop.current().start()
       In many cases, `tornado.web.Application.listen` can be used to avoid
       the need to explicitly create the `HTTPServer`.
    2. `~tornado.tcpserver.TCPServer.bind`/`~tornado.tcpserver.TCPServer.start`:
       simple multi-process::
            server = HTTPServer(app)
            server.bind(8888)
            server.start(0)  # Forks multiple sub-processes
            IOLoop.current().start()
       When using this interface, an `.IOLoop` must *not* be passed
       to the `HTTPServer` constructor.  `~.TCPServer.start` will always start
       the server on the default singleton `.IOLoop`.
    3. `~tornado.tcpserver.TCPServer.add_sockets`: advanced multi-process::
            sockets = tornado.netutil.bind_sockets(8888)
            tornado.process.fork_processes(0)
            server = HTTPServer(app)
            server.add_sockets(sockets)
            IOLoop.current().start()
       The `~.TCPServer.add_sockets` interface is more complicated,
       but it can be used with `tornado.process.fork_processes` to
       give you more flexibility in when the fork happens.
       `~.TCPServer.add_sockets` can also be used in single-process
       servers if you want to create your listening sockets in some
       way other than `tornado.netutil.bind_sockets`.
    .. versionchanged:: 4.0
       Added ``decompress_request``, ``chunk_size``, ``max_header_size``,
       ``idle_connection_timeout``, ``body_timeout``, ``max_body_size``
       arguments.  Added support for `.HTTPServerConnectionDelegate`
       instances as ``request_callback``.
    .. versionchanged:: 4.1
       `.HTTPServerConnectionDelegate.start_request` is now called with
       two arguments ``(server_conn, request_conn)`` (in accordance with the
       documentation) instead of one ``(request_conn)``.
    .. versionchanged:: 4.2
       `HTTPServer` is now a subclass of `tornado.util.Configurable`.
    """
    def __init__(self, *args, **kwargs):
        # Ignore args to __init__; real initialization belongs in
        # initialize since we're Configurable. (there's something
        # weird in initialization order between this class,
        # Configurable, and TCPServer so we can't leave __init__ out
        # completely)
        pass