from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
    return "Hello World!"

if __name__ == "__main__":
    app.run()

class Flask:
    def __init__(self, package_name):

        self.package_name = package_name 
        self.root_path = _get_package_path(self.package_name)

        self.view_functions = {}
        self.error_handlers = {}
        self.before_request_funcs = []
        self.after_request_funcs = []
        self.url_map = Map()
        
    def run(self, host='localhost', port=5000, **options):
        """Runs the application on a local development server.  If the
        :attr:`debug` flag is set the server will automatically reload
        for code changes and show a debugger in case an exception happened.
    
        :param host: the hostname to listen on.  set this to ``'0.0.0.0'``
                     to have the server available externally as well.
        :param port: the port of the webserver
        :param options: the options to be forwarded to the underlying
                        Werkzeug server.  See :func:`werkzeug.run_simple`
                        for more information.
        """
        from werkzeug import run_simple
        if 'debug' in options:
            self.debug = options.pop('debug')
        options.setdefault('use_reloader', self.debug)
        options.setdefault('use_debugger', self.debug)
        return run_simple(host, port, self, **options)
    
    def wsgi_app(self, environ, start_response):
        """The actual WSGI application.  This is not implemented in
        `__call__` so that middlewares can be applied:

            app.wsgi_app = MyMiddleware(app.wsgi_app)

        :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
        """
        with self.request_context(environ):
            rv = self.preprocess_request()
            if rv is None:
                rv = self.dispatch_request()
            response = self.make_response(rv)
            response = self.process_response(response)
            return response(environ, start_response)
    
    def __call__(self, environ, start_response):
        """Shortcut for :attr:`wsgi_app`"""
        return self.wsgi_app(environ, start_response)

class _RequestContext(object):
    """The request context contains all request relevant information.  It is
    created at the beginning of the request and pushed to the
    `_request_ctx_stack` and removed at the end of it.  It will create the
    URL adapter and request object for the WSGI environment provided.
    """

    def __init__(self, app, environ):
        self.app = app
        self.url_adapter = app.url_map.bind_to_environ(environ)
        self.request = app.request_class(environ)
        self.session = app.open_session(self.request)
        self.g = _RequestGlobals()
        self.flashes = None

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

    def __exit__(self, exc_type, exc_value, tb):
        # do not pop the request stack if we are in debug mode and an
        # exception happened.  This will allow the debugger to still
        # access the request object in the interactive shell.
        if tb is None or not self.app.debug:
            _request_ctx_stack.pop()
            
def request_context(self, environ):
    """Creates a request context from the given environment and binds
    it to the current context.  This must be used in combination with
    the `with` statement because the request is only bound to the
    current context for the duration of the `with` block.

    Example usage::

        with app.request_context(environ):
            do_something_with(request)

    :params environ: a WSGI environment
    """
    return _RequestContext(self, environ)

def preprocess_request(self):
    """Called before the actual request dispatching and will
    call every as :meth:`before_request` decorated function.
    If any of these function returns a value it's handled as
    if it was the return value from the view and further
    request handling is stopped.
    """
    for func in self.before_request_funcs:
        rv = func()
        if rv is not None:
            return rv

def before_request(self, f):
    """Registers a function to run before each request."""
    self.before_request_funcs.append(f)
    return f

def dispatch_request(self):
    """Does the request dispatching.  Matches the URL and returns the
    return value of the view or error handler.  This does not have to
    be a response object.  In order to convert the return value to a
    proper response object, call :func:`make_response`.
    """
    try:
        endpoint, values = self.match_request()
        return self.view_functions[endpoint](**values)
    except HTTPException as e:
        handler = self.error_handlers.get(e.code)
        if handler is None:
            return e
        return handler(e)
    except Exception as e:
        handler = self.error_handlers.get(500)
        if self.debug or handler is None:
            raise
        return handler(e)

def match_request(self):
    """Matches the current request against the URL map and also
    stores the endpoint and view arguments on the request object
    is successful, otherwise the exception is stored.
    """
    rv = _request_ctx_stack.top.url_adapter.match()
    request.endpoint, request.view_args = rv
    return rv

def route(self, rule, **options):
    def decorator(f):
        self.add_url_rule(rule, f.__name__, **options)
        self.view_functions[f.__name__] = f
        return f
    return decorator
    
def add_url_rule(self, rule, endpoint, **options):
    options['endpoint'] = endpoint
    options.setdefault('methods', ('GET',))
    self.url_map.add(Rule(rule, **options))

# context locals
_request_ctx_stack = LocalStack()
current_app = LocalProxy(lambda: _request_ctx_stack.top.app)
request = LocalProxy(lambda: _request_ctx_stack.top.request)
session = LocalProxy(lambda: _request_ctx_stack.top.session)
g = LocalProxy(lambda: _request_ctx_stack.top.g)

class LocalStack(object):

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

    def push(self, obj):
        rv = getattr(self._local, 'stack', None)
        if rv is None:
            self._local.stack = rv = []
        rv.append(obj)
        return rv

    def pop(self):
        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

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

    def __init__(self):
        # 数据保存在 __storage__ 中，后续访问都是对该属性的操作
        object.__setattr__(self, '__storage__', {})
        object.__setattr__(self, '__ident_func__', get_ident)

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

    # 清空当前线程/协程保存的所有数据
    def __release_local__(self):
        self.__storage__.pop(self.__ident_func__(), None)

    # 下面三个方法实现了属性的访问、设置和删除。
    # 注意到，内部都调用 `self.__ident_func__` 获取当前线程或者协程的 id，然后再访问对应的内部字典。
    # 如果访问或者删除的属性不存在，会抛出 AttributeError。
    # 这样，外部用户看到的就是它在访问实例的属性，完全不知道字典或者多线程/协程切换的实现
    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)