flask.py

# -*- coding: utf-8 -*-
"""
    flask
    ~~~~~

    A microframework based on Werkzeug.  It's extensively documented
    and follows best practice patterns.

    :copyright: (c) 2010 by Armin Ronacher.
    :license: BSD, see LICENSE for more details.


    flask 0.1 的注释版，里面涉及一些具体概念、术语请通过
    http://blog.guoweikuang.com 博客查找具体内容
"""
from __future__ import with_statement
import os
import sys

from threading import local
from jinja2 import Environment, PackageLoader, FileSystemLoader
from werkzeug import Request as RequestBase, Response as ResponseBase, \
     LocalStack, LocalProxy, create_environ, cached_property, \
     SharedDataMiddleware
from werkzeug.routing import Map, Rule
from werkzeug.exceptions import HTTPException, InternalServerError
from werkzeug.contrib.securecookie import SecureCookie

# utilities we import from Werkzeug and Jinja2 that are unused
# in the module but are exported as public interface.
from werkzeug import abort, redirect
from jinja2 import Markup, escape

# use pkg_resource if that works, otherwise fall back to cwd.  The
# current working directory is generally not reliable with the notable
# exception of google appengine.
try:
    import pkg_resources
    pkg_resources.resource_stream
except (ImportError, AttributeError):
    pkg_resources = None


class Request(RequestBase):
    """The request object used by default in flask.  Remembers the
    matched endpoint and view arguments.

    It is what ends up as :class:`~flask.request`.  If you want to replace
    the request object used you can subclass this and set
    :attr:`~flask.Flask.request_class` to your subclass.

    该类主要使用了Werkzeug库中的Request类实现的
    flask 默认的请求对象. 保存了匹配的端点(博客里该部分解释)和视图参数.
    想要替换默认的request对象，可以子类化这个类，然后赋值给Flask中的request_class.
    """

    def __init__(self, environ):
        RequestBase.__init__(self, environ)
        self.endpoint = None    # 当前request的端点
        self.view_args = None   # 当前request的视图参数(作为关键字参数传入视图函数)


class Response(ResponseBase):
    """The response object that is used by default in flask.  Works like the
    response object from Werkzeug but is set to have a HTML mimetype by
    default.  Quite often you don't have to create this object yourself because
    :meth:`~flask.Flask.make_response` will take care of that for you.

    If you want to replace the response object used you can subclass this and
    set :attr:`~flask.Flask.request_class` to your subclass.

    该类主要使用了Werkzeug库中的Response类实现的.
    flask 默认的响应对象，这里设置了默认的mimetype为HTML类型（即是定义了Content-Type 返回类型为HTML).
    不用自己来创建这个对象，因为Flask类中有make_response 会创建它.
    如果想要替换该response对象，你可以子类化这个Response类，然后通过赋值给Flask的request_class来替换.
    """
    default_mimetype = 'text/html'


class _RequestGlobals(object):
    pass


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 url_for(endpoint, **values):
    """Generates a URL to the given endpoint with the method provided.

    :param endpoint: the endpoint of the URL (name of the function)
    :param values: the variable arguments of the URL rule
    """
    return _request_ctx_stack.top.url_adapter.build(endpoint, values)


def flash(message):
    """Flashes a message to the next request.  In order to remove the
    flashed message from the session and to display it to the user,
    the template has to call :func:`get_flashed_messages`.

    :param message: the message to be flashed.
    """
    session['_flashes'] = (session.get('_flashes', [])) + [message]


def get_flashed_messages():
    """Pulls all flashed messages from the session and returns them.
    Further calls in the same request to the function will return
    the same messages.
    """
    flashes = _request_ctx_stack.top.flashes
    if flashes is None:
        _request_ctx_stack.top.flashes = flashes = \
            session.pop('_flashes', [])
    return flashes


def render_template(template_name, **context):
    """Renders a template from the template folder with the given
    context.

    :param template_name: the name of the template to be rendered
    :param context: the variables that should be available in the
                    context of the template.
    """
    current_app.update_template_context(context)
    return current_app.jinja_env.get_template(template_name).render(context)


def render_template_string(source, **context):
    """Renders a template from the given template source string
    with the given context.

    :param template_name: the sourcecode of the template to be
                          rendered
    :param context: the variables that should be available in the
                    context of the template.
    """
    current_app.update_template_context(context)
    return current_app.jinja_env.from_string(source).render(context)


def _default_template_ctx_processor():
    """Default template context processor.  Injects `request`,
    `session` and `g`.
    """
    reqctx = _request_ctx_stack.top
    return dict(
        request=reqctx.request,
        session=reqctx.session,
        g=reqctx.g
    )


def _get_package_path(name):
    """Returns the path to a package or cwd if that cannot be found."""
    try:
        return os.path.abspath(os.path.dirname(sys.modules[name].__file__))
    except (KeyError, AttributeError):
        return os.getcwd()


class Flask(object):
    """The flask object implements a WSGI application and acts as the central
    object.  It is passed the name of the module or package of the
    application.  Once it is created it will act as a central registry for
    the view functions, the URL rules, template configuration and much more.

    The name of the package is used to resolve resources from inside the
    package or the folder the module is contained in depending on if the
    package parameter resolves to an actual python package (a folder with
    an `__init__.py` file inside) or a standard module (just a `.py` file).

    For more information about resource loading, see :func:`open_resource`.

    Usually you create a :class:`Flask` instance in your main module or
    in the `__init__.py` file of your package like this::

        from flask import Flask
        app = Flask(__name__)
    """

    #: the class that is used for request objects.  See :class:`~flask.request`
    #: for more information.
    request_class = Request

    #: the class that is used for response objects.  See
    #: :class:`~flask.Response` for more information.
    response_class = Response

    #: path for the static files.  If you don't want to use static files
    #: you can set this value to `None` in which case no URL rule is added
    #: and the development server will no longer serve any static files.
    static_path = '/static'

    #: if a secret key is set, cryptographic components can use this to
    #: sign cookies and other things.  Set this to a complex random value
    #: when you want to use the secure cookie for instance.
    secret_key = None

    #: The secure cookie uses this for the name of the session cookie
    session_cookie_name = 'session'

    #: options that are passed directly to the Jinja2 environment
    jinja_options = dict(
        autoescape=True,
        extensions=['jinja2.ext.autoescape', 'jinja2.ext.with_']
    )

    def __init__(self, package_name):
        #: the debug flag.  Set this to `True` to enable debugging of
        #: the application.  In debug mode the debugger will kick in
        #: when an unhandled exception ocurrs and the integrated server
        #: will automatically reload the application if changes in the
        #: code are detected.
        self.debug = False

        #: the name of the package or module.  Do not change this once
        #: it was set by the constructor.
        self.package_name = package_name

        #: where is the app root located?
        self.root_path = _get_package_path(self.package_name)

        #: a dictionary of all view functions registered.  The keys will
        #: be function names which are also used to generate URLs and
        #: the values are the function objects themselves.
        #: to register a view function, use the :meth:`route` decorator.
        self.view_functions = {}

        #: a dictionary of all registered error handlers.  The key is
        #: be the error code as integer, the value the function that
        #: should handle that error.
        #: To register a error handler, use the :meth:`errorhandler`
        #: decorator.
        self.error_handlers = {}

        #: a list of functions that should be called at the beginning
        #: of the request before request dispatching kicks in.  This
        #: can for example be used to open database connections or
        #: getting hold of the currently logged in user.
        #: To register a function here, use the :meth:`before_request`
        #: decorator.
        self.before_request_funcs = []

        #: a list of functions that are called at the end of the
        #: request.  Tha function is passed the current response
        #: object and modify it in place or replace it.
        #: To register a function here use the :meth:`after_request`
        #: decorator.
        self.after_request_funcs = []

        #: a list of functions that are called without arguments
        #: to populate the template context.  Each returns a dictionary
        #: that the template context is updated with.
        #: To register a function here, use the :meth:`context_processor`
        #: decorator.
        self.template_context_processors = [_default_template_ctx_processor]

        self.url_map = Map()

        if self.static_path is not None:
            self.url_map.add(Rule(self.static_path + '/<filename>',
                                  build_only=True, endpoint='static'))
            if pkg_resources is not None:
                target = (self.package_name, 'static')
            else:
                target = os.path.join(self.root_path, 'static')
            self.wsgi_app = SharedDataMiddleware(self.wsgi_app, {
                self.static_path: target
            })

        #: the Jinja2 environment.  It is created from the
        #: :attr:`jinja_options` and the loader that is returned
        #: by the :meth:`create_jinja_loader` function.
        self.jinja_env = Environment(loader=self.create_jinja_loader(),
                                     **self.jinja_options)
        self.jinja_env.globals.update(
            url_for=url_for,
            get_flashed_messages=get_flashed_messages
        )

    def create_jinja_loader(self):
        """Creates the Jinja loader.  By default just a package loader for
        the configured package is returned that looks up templates in the
        `templates` folder.  To add other loaders it's possible to
        override this method.
        """
        if pkg_resources is None:
            return FileSystemLoader(os.path.join(self.root_path, 'templates'))
        return PackageLoader(self.package_name)

    def update_template_context(self, context):
        """Update the template context with some commonly used variables.
        This injects request, session and g into the template context.

        :param context: the context as a dictionary that is updated in place
                        to add extra variables.
        """
        reqctx = _request_ctx_stack.top
        for func in self.template_context_processors:
            context.update(func())

    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.

        在本地的开发服务器运行应用.如果设置了Flask 的debug，当项目代码发生变化时，
        自动重载，并在异常发生时提供一个调试器.

        :param host: the hostname to listen on.  set this to ``'0.0.0.0'``
                     to have the server available externally as well.
                    
                     需要监听的hostname, 设置为‘0.0.0.0’ 外部服务器可访问

        :param port: the port of the webserver

                     web服务的端口

        :param options: the options to be forwarded to the underlying
                        Werkzeug server.  See :func:`werkzeug.run_simple`
                        for more information.

                     这些可选项会转发给Werkzeug服务器处理.
        """
        from werkzeug import run_simple   # 使用werkzeug 的run_simple 来运行wsgi应用, 具体请看博客
        if 'debug' in options:
            self.debug = options.pop('debug')
        options.setdefault('use_reloader', self.debug)  # 如果debug == True, 开启重载器，也就是监控代码是否变化
        options.setdefault('use_debugger', self.debug)  # 如果debug == True，开启调试器, 在异常时提供
        return run_simple(host, port, self, **options)

    def test_client(self):
        """Creates a test client for this application.  For information
        about unit testing head over to :ref:`testing`.
        """
        from werkzeug import Client
        return Client(self, self.response_class, use_cookies=True)

    def open_resource(self, resource):
        """Opens a resource from the application's resource folder.  To see
        how this works, consider the following folder structure::

            /myapplication.py
            /schemal.sql
            /static
                /style.css
            /template
                /layout.html
                /index.html

        If you want to open the `schema.sql` file you would do the
        following::

            with app.open_resource('schema.sql') as f:
                contents = f.read()
                do_something_with(contents)

        :param resource: the name of the resource.  To access resources within
                         subfolders use forward slashes as separator.
        """
        if pkg_resources is None:
            return open(os.path.join(self.root_path, resource), 'rb')
        return pkg_resources.resource_stream(self.package_name, resource)

    def open_session(self, request):
        """Creates or opens a new session.  Default implementation stores all
        session data in a signed cookie.  This requires that the
        :attr:`secret_key` is set.

        :param request: an instance of :attr:`request_class`.
        """
        key = self.secret_key
        if key is not None:
            return SecureCookie.load_cookie(request, self.session_cookie_name,
                                            secret_key=key)

    def save_session(self, session, response):
        """Saves the session if it needs updates.  For the default
        implementation, check :meth:`open_session`.

        :param session: the session to be saved (a
                        :class:`~werkzeug.contrib.securecookie.SecureCookie`
                        object)
        :param response: an instance of :attr:`response_class`
        """
        if session is not None:
            session.save_cookie(response, self.session_cookie_name)

    def add_url_rule(self, rule, endpoint, **options):
        """Connects a URL rule.  Works exactly like the :meth:`route`
        decorator but does not register the view function for the endpoint.

        连接一个URL规则. 类似于route装饰器做的工作，但是不会为端点（endpoint)注册视图函数

        Basically this example::

            @app.route('/')
            def index():
                pass

        Is equivalent to the following::

            def index():
                pass
            app.add_url_rule('index', '/')
            app.view_functions['index'] = index

            这里示例错了，应该是
            app.add_url_rule('/', 'index')
            端点(endpoint)是视图函数名称, rule（规则)是'/'.


        :param rule: the URL rule as string    # 字符串类型的URL规则
        :param endpoint: the endpoint for the registered URL rule.  Flask
                         itself assumes the name of the view function as
                         endpoint

                         已注册URL规则的端点. Flask 本身将视图函数的名称作为一个端点.
        :param options: the options to be forwarded to the underlying
                        :class:`~werkzeug.routing.Rule` object

                        可选项，转发给底层的werkzeug.routing.Rule对象的选项
        """
        options['endpoint'] = endpoint  
        options.setdefault('methods', ('GET',))  # 默认设置methods为GET
        self.url_map.add(Rule(rule, **options))  # 使用Werkzeug的Rule来实现规则的相关工作，具体参考博客.

    def route(self, rule, **options):
        """A decorator that is used to register a view function for a
        given URL rule.  
        
        用于为给定的URL 规则注册一个视图函数的装饰器.
        
        Example::

            @app.route('/')
            def index():
                return 'Hello World'

        Variables parts in the route can be specified with angular
        brackets (``/user/<username>``).  By default a variable part
        in the URL accepts any string without a slash however a different
        converter can be specified as well by using ``<converter:name>``.

        可以使用<>尖括号指定route路由的变量部分，默认情况下URL中的变量
        接收任何的没有斜杠的字符串，但是可以使用<converter:name>来指定不同的converter.

        Variable parts are passed to the view function as keyword
        arguments.

        变量部分作为视图函数的关键字参数传入.

        转换器(convertor) 类型可以如下几种：
        The following converters are possible:

        =========== ===========================================
        `int`       accepts integers
        `float`     like `int` but for floating point values
        `path`      like the default but also accepts slashes
        =========== ===========================================

        Here some examples::

            @app.route('/')
            def index():
                pass

            @app.route('/<username>')
            def show_user(username):
                pass

            @app.route('/post/<int:post_id>')
            def show_post(post_id):
                pass

        An important detail to keep in mind is how Flask deals with trailing
        slashes.  The idea is to keep each URL unique so the following rules
        apply:

        一个重要的细节需要注意的是,Flask在处理斜线的. 为了保持URL的唯一性，下面的规则会被应用：

        1. If a rule ends with a slash and is requested without a slash
           by the user, the user is automatically redirected to the same
           page with a trailing slash attached.

        1、如果一个rule以一个斜线结尾并且用户请求路径中没有带上斜线，那么用户
        自动重定向到相同的已经附加了斜线的页面。

        2. If a rule does not end with a trailing slash and the user request
           the page with a trailing slash, a 404 not found is raised.

        2、如果一个rule不以一个斜线结尾并且用户请求路径中带上这个斜线的话，
        一个404 not found 错误会报出来.

        This is consistent with how web servers deal with static files.  This
        also makes it possible to use relative link targets safely.

        The :meth:`route` decorator accepts a couple of other arguments
        as well:

        :param rule: the URL rule as string  # 字符串类型的URL 规则
        :param methods: a list of methods this rule should be limited
                        to (``GET``, ``POST`` etc.).  By default a rule
                        just listens for ``GET`` (and implicitly ``HEAD``).

                        这个rule(规则)应该限制一系列的methods(GET, POST).
                        默认规则就是只监听GET请求.
        :param subdomain: specifies the rule for the subdoain in case
                          subdomain matching is in use.

                         当子域名匹配使用时，为规则指定子域.
        :param strict_slashes: can be used to disable the strict slashes
                               setting for this rule.  See above.

                               是否为这个rule关闭严格的斜杠匹配.
        :param options: other options to be forwarded to the underlying
                        :class:`~werkzeug.routing.Rule` object.
        """
        def decorator(f):
            """
            这是最简单的装饰器的实践，使用add_url_rule将rule(路由规则)与视图函数绑定在一起.
            """
            self.add_url_rule(rule, f.__name__, **options)  # 添加路由规则,options传入一些可选项给Rule类
            self.view_functions[f.__name__] = f             # 将端点（endpoint, 默认使用函数名f.__name__)与函数对象的映射存入前面Flask定义的view_functions列表中
            return f
        return decorator

    def errorhandler(self, code):
        """A decorator that is used to register a function give a given
        error code.  Example::

            @app.errorhandler(404)
            def page_not_found():
                return 'This page does not exist', 404

        You can also register a function as error handler without using
        the :meth:`errorhandler` decorator.  The following example is
        equivalent to the one above::

            def page_not_found():
                return 'This page does not exist', 404
            app.error_handlers[404] = page_not_found

        :param code: the code as integer for the handler
        """
        def decorator(f):
            self.error_handlers[code] = f
            return f
        return decorator

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

    def after_request(self, f):
        """Register a function to be run after each request."""
        self.after_request_funcs.append(f)
        return f

    def context_processor(self, f):
        """Registers a template context processor function."""
        self.template_context_processors.append(f)
        return f

    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 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, e:
            handler = self.error_handlers.get(e.code)
            if handler is None:
                return e
            return handler(e)
        except Exception, e:
            handler = self.error_handlers.get(500)
            if self.debug or handler is None:
                raise
            return handler(e)

    def make_response(self, rv):
        """Converts the return value from a view function to a real
        response object that is an instance of :attr:`response_class`.

        The following types are allowd for `rv`:

        ======================= ===========================================
        :attr:`response_class`  the object is returned unchanged
        :class:`str`            a response object is created with the
                                string as body
        :class:`unicode`        a response object is created with the
                                string encoded to utf-8 as body
        :class:`tuple`          the response object is created with the
                                contents of the tuple as arguments
        a WSGI function         the function is called as WSGI application
                                and buffered as response object
        ======================= ===========================================

        :param rv: the return value from the view function
        """
        if isinstance(rv, self.response_class):
            return rv
        if isinstance(rv, basestring):
            return self.response_class(rv)
        if isinstance(rv, tuple):
            return self.response_class(*rv)
        return self.response_class.force_type(rv, request.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 process_response(self, response):
        """Can be overridden in order to modify the response object
        before it's sent to the WSGI server.  By default this will
        call all the :meth:`after_request` decorated functions.

        :param response: a :attr:`response_class` object.
        :return: a new response object or the same, has to be an
                 instance of :attr:`response_class`.
        """
        session = _request_ctx_stack.top.session
        if session is not None:
            self.save_session(session, response)
        for handler in self.after_request_funcs:
            response = handler(response)
        return response

    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 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 test_request_context(self, *args, **kwargs):
        """Creates a WSGI environment from the given values (see
        :func:`werkzeug.create_environ` for more information, this
        function accepts the same arguments).
        """
        return self.request_context(create_environ(*args, **kwargs))

    def __call__(self, environ, start_response):
        """Shortcut for :attr:`wsgi_app`"""
        return self.wsgi_app(environ, start_response)


# 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)