flask/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.
"""
import os
import sys
import pkg_resources
from threading import local
from contextlib import contextmanager
from jinja2 import Environment, PackageLoader
from werkzeug import Request, Response, LocalStack, LocalProxy, \
     create_environ, cached_property
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


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

    def __init__(self, environ):
        Request.__init__(self, environ)
        self.endpoint = None
        self.view_args = None


class FlaskResponse(Response):
    """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.
    """
    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 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`.
    """
    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.
    """
    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.
    """
    current_app.update_template_context(context)
    return current_app.jinja_env.from_string(source).render(context)


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
    request_class = FlaskRequest

    #: the class that is used for response objects
    response_class = FlaskResponse

    #: 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

        #: 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:`request_init`
        #: decorator.
        self.request_init_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:`request_shtdown`
        #: decorator.
        self.request_shutdown_funcs = []
        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'))

        #: 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.
        """
        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.
        """
        reqctx = _request_ctx_stack.top
        context['request'] = reqctx.request
        context['session'] = reqctx.session
        context['g'] = reqctx.g

    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.
        """
        from werkzeug import run_simple
        if 'debug' in options:
            self.debug = options.pop('debug')
        if self.static_path is not None:
            options['static_files'] = {
                self.static_path:   (self.package_name, 'static')
            }
        options.setdefault('use_reloader', self.debug)
        options.setdefault('use_debugger', self.debug)
        return run_simple(host, port, self, **options)

    @cached_property
    def test(self):
        """A test client for this application"""
        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)
        """
        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.
        """
        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`.
        """
        if session is not None:
            session.save_cookie(response, self.session_cookie_name)

    def route(self, rule, **options):
        """A decorator that is used to register a view function for a
        given URL rule.  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 differnt
        converter can be specified as well by using ``<converter:name>``.

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

        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:

        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.
        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.

        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 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``).
        :param subdomain: specifies the rule for the subdoain in case
                          subdomain matching is in use.
        :param strict_slashes: can be used to disable the strict slashes
                               setting for this rule.  See above.
        """
        def decorator(f):
            if 'endpoint' not in options:
                options['endpoint'] = f.__name__
            options.setdefault('methods', ('GET',))
            self.url_map.add(Rule(rule, **options))
            self.view_functions[options['endpoint']] = f
            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
        """
        def decorator(f):
            self.error_handlers[code] = f
            return f
        return decorator

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

    def request_shutdown(self, f):
        """Register a function to be run after each request."""
        self.request_shutdown_funcs.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`.
        """
        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 :func:`request_init` 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.request_init_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.
        """
        session = _request_ctx_stack.top.session
        if session is not None:
            self.save_session(session, response)
        for handler in self.request_shutdown_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)
        """
        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)

    @contextmanager
    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)
        """
        _request_ctx_stack.push(_RequestContext(self, environ))
        try:
            yield
        finally:
            _request_ctx_stack.pop()

    def test_request_context(self, *args, **kwargs):
        """Creates a WSGI environment from the given values (see
        :func:`werkzeug.create_environ` for more information).
        """
        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)